During discussion on the GURPS forums, I realized there's a way to make things a bit simpler.
Starting MP: Characters start with 20 MP. Optionally, characters can start with more MP, at  per +1. Less MP might also be an option, similarly at [-10] per -1. Limits are up to the GM; +10 is suggested.
Bonus MP: Characters with high skills and defenses can gain additional MP by opting not to use them. A character who chooses not to invest any MP in Attack or Aim gains 1 Bonus MP for every 2 points their best melee or ranged skill is above 14. Such Bonus MP can be invested in Defense, Awareness, Evaluate, or Ready.
A character who instead chooses not to invest any MP in Defense gains 2 Bonus MP for every point his Dodge is above 9. He also gains 1 Bonus MP for every point his best defense is above Dodge or 9, whichever is higher. Such Bonus MP can be invested in anything but Defense, Awareness, or Evaluate.
Optionally, if the character has higher Melee skill than Ranged, his Bonus MP is based on Ranged skill, and he also has Melee MP equal to the difference. Melee MP only boosts Defense against melee attacks, Awareness only gives a bonus within Per yards, Evaluate works normally, and Ready only works melee weapons. If the character has higher Ranged skill than melee, his Bonus MP is based on Ranged skill, and he also has Ranged MP equal to the difference. Ranged MP only boosts Defense against ranged attacks, Awareness works normally, Evaluate doesn't work at all, and Ready only applies to ranged weapons (and reloading them).
Similarly, it may be appropriate to not allow Bonus MP from Parry or Block to improve Attack, Damage, or Aim for ranged attacks
Trading FP for MP: A character can exchange FP for MP at any point, at a rate of 4 MP per FP. This replaces the normal rules for Extra Effort in Combat. If using The Last Gasp, you may instead exchange AP for MP.
Acrobatics MP: Once per turn, at any point, a character may attempt to use Acrobatics or similar to gain additional MP. On a Success, the character gains 2x(MoS), minimum 1, MP. On a Failure, the character loses 2x(MoF) MP. Gained MP can be spent to negate Attack penalties for Acrobatic Attack, to enhance defense (in the manner of Acrobatic Dodge), or for Movement. Lost MP from a Failure can be reclaimed by lowering the character's Posture - going from standing/crouching to sitting/kneeling or sitting/kneeling to prone/supine recovers 4 MP, while going form standing/crouching to prone/supine recovers 8 MP. If a character has already allocated his MP for the round when making this attempt (such as for an Acrobatic Dodge), lost MP is taken from Defense. If falling prone/supine and losing the ability to defend is insufficient to offset the MP loss, the character suffers a pratfall and must roll against HT+5 - Success means he is Stunned, Failure means he is rendered unconscious. An already Stunned character doesn't get a roll here, and is simply rendered unconscious.
Stunning: A character who is Stunned is at 1/4 MP (base 5). Bonus MP from Acrobatics or forgoing attacks/defenses (see later) are similarly reduced, but those from spending FP/AP are not. He can still spend MP normally, however. This allows him to attempt weak attacks, clumsy defenses, or stumble away at reduced Move.
MP may be spent as follows.
Attack: Making an attack costs 6 MP, which allows an attack at -4 to skill. Each additional MP gives a +1 to attack, to a maximum of +4 for melee attacks; ranged attacks are limited to a maximum of +2. Optionally, every 2 points above skill 14 increases this cap by +1 for melee attacks; ranged attacks are instead +1 per 4 points above skill 14. MP can also be spent to reduce or negate penalties other than those resulting from Range or use of Deceptive Attack, Setup Attack, Predictive Shot, or similar - every 2 MP negates up to -3 in penalties. Penalties for hit locations and similar are still limited to 1/2 the initial penalty being negated, however. For melee attacks, you may add one - and only one - extra yard of Reach to your attack for 4 MP (optionally, use "A Matter of Inches," with each step above Average being -1 MP to cost, each step below Average being +1 MP to cost). For a ranged attack to benefit from a previous Aim, you must spend an additional 2 MP.
Note the above is a one-time cost - to get additional attacks, use Dual Weapon Attack, Rapid Strike, etc, at the normal penalties. MP invested to reduce penalties can eliminate these as well.
Damage: If making an attack, damage can be improved. Every 2 MP is +1 or +0.5/die to damage, whichever is better. This cannot exceed +4 (8 MP) for an attack dealing 1d+n damage, or +3.5 (14 MP) for an attack dealing 2d+n damage or more. Additionally, melee impaling weapons that strike a target can be purposefully wrenched out to cause additional damage. This costs at least 1 MP and calls for an attack roll, at -4 relative to the first. This can be defended against normally, with a successful defense meaning the weapon is pulled out without further injury, but the defense suffers the full Shock penalty of the initial injury. Swing impaling weapons are harder to do this with, due to poor leverage, calling for an additional -3 to skill (total -7). Barbed weapons are also harder to remove, suffering the same -3. Every MP results in 20% of the initial injury being suffered as the weapon is pulled out, to a maximum of 60% for 3 MP. For barbed weapons, this is instead 30% per MP, to a maximum of 90% for 3 MP. For swing impaling and barbed weapons, dealing 60% or more injury prevents them from being stuck in the target.
The character can reduce the MP cost for attacking by dealing less damage. This is 5 MP for -1/die, 4 MP for -2/die, and 3 MP for causing no damage. A no damage attack can be a tap (for delivering touch effects) or a Feint or similar. For characters making multiple attacks, this damage reduction must apply to all of them to get the discount.
Defense: Being able to defend costs 6 MP, which allows defenses at -2. 8 MP increases this to -1, and 10 MP allows full defense. Following this, you may improve defenses, at a rate of 2 MP per +1 to a single use of a defense, 4 MP per +1 to all uses of a given defense, or 8 MP per +1 to all defenses. You may also spend 4 MP for a single use of a Double Defense, or 8 MP to make all defenses for the round Double Defenses (a Double Defense is where you get to roll two different defenses against a single attack).
A character who starts with less than 5 MP, due to Stun or similar, can opt to spend all MP on defense, allowing defenses at -4.
Movement: You get a free Step or Retreat. Each additional Step costs 2 MP, to a maximum of half Move (the GM may extend this to full move for chambara and similar games). Simply moving up to half Move costs 6 MP, while moving up to full Move costs 10 MP. If making an attack with a ranged weapon, half Move results in an attack penalty equal to half Bulk, while full Move results in an attack penalty equal to Bulk. Changing Posture costs 10 MP to go a single step (prone->kneeling, kneeling-crouching/standing), 20 MP to go two steps (prone->crouching/standing).
Awareness: If using Situational Awareness from Tactical Shooting, the default is -4 to the roll for 0 MP (a glance). Every 2 MP is worth +1 to this, to a maximum of +8 or (skill/2+1), whichever is better. "Skill" here is the best of Perception, Observation, Per-based Soldier, and Per-based Tactics.
Recovery: If using The Last Gasp, it costs 2 MP to attempt to recover AP. This allows for an HT-4 roll. Each additional MP is a +1 to this roll.
Aim: If using Take Aim, resolve this as an attack (but with no option to increase or reduce Damage). If not, Aim costs 10 MP.
Concentrate: For actions that require a roll, treat this much like Attack, above. For actions that require multiple rounds of Concentrate, followed by a roll, you must spend at least 6 MP each round on Concentrate, and roll based on the average MP spent (it's easiest to just maintain the same cost each round, however). For actions that don't require a roll, Concentrate costs 10 MP.
Evaluate: Every 3 MP spent on Evaluate negates -1 in penalties from Feints, Deceptive Attacks, etc from the target in question over the round, and also give you an additional 1 MP to spend only against that target next round. Such Evaluate MP are limited to the greatest of Melee MP, Ranged MP, and Dodge MP.
Ready: As Concentrate, above. If Fast-Draw reduces this to a free action, it costs 0 MP.
Wait: A character can opt to delay his turn with a specific trigger, as usual. Optionally, he must dictate his Combat Posture (where he is spending his MP) upon declaring the Wait.
Redistributing MP: A character can reclaim "overspent" MP and spend it elsewhere (typically on either Defense or Evaluate, but Awareness may also be an option). If the character was prevented from using some amount of MP, he can reclaim up to half of it. If he did use it, but had a higher MoS than needed, he can reclaim up to a third of the excess. Note this latter option essentially retroactively drops your effective skill, so you may not want to do it if you expect to make further rolls against that skill, and you can only reclaim points based on your lowest such roll during the round.
Let's rerun our example character through the paces. Once again, he's a rifle-toting TL 5 mage named Jack, with ST 13, HT 12, Dodge 11 and Move 7. His best melee skill is Spear 16 (for bayonet fighting), his best ranged skill is Guns(Longarm) 18, he has Acrobatics 14, Path of Energy 20, and Fast Draw (Ammo) 18. His best Awareness skill is Per-based Soldier, at 14. He also has Bayoneteer (no Guns penalty for having an attached bayonet), Combat Reflexes, Ritual Adept, Ritual Mastery (MTC Shot), and Quick Reload (Muzzleloading Rifle). He could also have a Hold Spell Perk (letting him finish gathering energy for a ritual, then wait to actually cast it, rather than doing so immediately), but I think that functionality is balanced enough to be the RPM default.
He starts with 20 MP, as normal. If he opts not to attack, he has Bonus MP 1, Ranged MP 1. If he chooses not to defend, he has Bonus MP 2, Melee MP 1.
At base, Jack can attack with his bayonet at skill 12 or his rifle at skill 14 for 6 MP, Dodge at 9 and Parry at 10 for 6 MP, or move up to 4 yards for 6 MP. If not spending anything on Awareness, he rolls against 12 there. He can attempt to recover AP with a roll against 8 (HT-4) for 2 MP. He can boost Spear up to Skill+5, or Guns(Longarm) up to Skill+6. He's at Dodge 10 Parry 11 for 8 MP, Dodge 11 Parry 12 for 10 MP, after which he uses the normal (2 MP per single +1, 4 MP per outright +1 to single defense type, 8 MP per +1 to all defenses).
Let's say Jack is in a long battle, and has exhausted his supply of MTC Shot Charms (MTC Shot is a 30 energy ritual that enhances the burn of gunpowder, markedly increasing damage). He gets behind some cover and starts loading a normal bullet while gathering energy for MTC Shot. Reloading with Fast Draw is going to take him 12 seconds. As he's not going to Attack, he gets 1 Bonus MP and one Ranged MP, in addition to his base 20 MP. He spends 6 on Concentrate, 8 (6 base, 1 bonus, 1 ranged) on Ready, and 8 on defense. He continues this for 5 rounds without issues, calling for a roll against MTC Shot. With only 6 MP invested in it, he's rolling against an 18. He gets a 10, gathering 8 energy.
He continues on, but when he rolls an 8 against his Awareness 12, he notices a sorcery-twisted, misshapen husk of a man charging him with an oversized sword. Rather than fight with his back to the wall, he temporarily stops reloading, continues spending 6 MP on his ritual, spends 2 MP for an extra Step, spends 9 (7+2) on Defense, and the remaining 7 on an attack, charging and thrusting at the creature at skill 13. He scores a hit, but it bats his weapon aside and steps in with a swing. He burns 1 FP, spending 2 MP on getting a +1 to this Dodge, and 2 MP on getting an extra Step, which he uses to Retreat, for an effective Dodge 14, letting him get out of the creature's way. Some bullets are hitting the ground near him, so he knows he needs to end this quickly and get back to cover before his foes decide to aim first, so he eschews defense this next round, gaining Bonus MP 1 and Melee MP 1. He spends 6 MP on Concentrate, 10 MP (8+2) on his attack, 2 to increase damage, 2 to rip the bayonet out for still more damage (assuming a hit), and 2 to negate -3 of the penalty to stab the thing in its exposed neck. That's an attack at 16 (seriously screwed up my math in the previous example), with a success dealing 1d+4 (2d) damage and allowing for an attack at 12, with success dealing a further 40% of initial damage. He opts for a -4/-2 Deceptive Attack, for skill 12, and he rolls a 7 on his attack The target fails its defense, and he rolls for damage - 10. This becomes 20 injury, and he rolls against 12 (opts against going deceptive), rolling an 11. Thanks to the -4 from shock, the creature fails its defense against this, and suffers a further 8 injury. It fails its Death Check markedly, and the GM rules Jack very nearly decapitated it ripping his blade out. Jack uses the free step remaining in his maneuver to get back into cover.
The rest of his time reloading and gathering energy is uneventful - on second 10 he rolls a 9, gathering 9 energy. On second 14 (he lost 2 seconds killing his attacker) he finishes reloading - as each second he spend actually reloading was with 8 MP, he rolls against 16 to reload, and succeeds - his rifle is ready for shooting. Without reloading eating up his concentration, he puts the 12 MP into Concentrate, 10 (8+2) into Defense, for Dodge 11 against ranged attacks (Dodge 10, Parry 11 against melee). He gets another roll against Path of Energy, which spent 4 seconds of the last 5 with skill 18, 1 second with skill 24, for an average of skill 19, which gets reduced to 18 for being the third gathering attempt - a 12 means 6 more energy (running total is 23). Figuring he'll have the spell ready soon, he changes things up again - 8 in Concentrate, for skill 20, 10 (8+2) in Defense, for 11 Dodge/Parry, and 4 in Awareness, boosting that to 14, so he can try to find someone worth shooting. By the end of those 5 seconds, he's located an enemy mage protected by more of those misshapen men, and is ready to roll to gather again. Effective skill is 19 (again, -1 for beyond beyond the 3rd gathering attempt), and he rolls a 10 - enough to complete the ritual.
With the ritual ready to go, he takes aim at his target. He spends 10 on Defense and 10 on Aim, letting him do so at full skill (minus half range). He succeeds, and rolls well enough that he gets his full Acc (4) against the target. He's in a decent defensive position, and he has his eyes on a high-value target, so he eschews defenses for the follow up attack (for +2 MP). He needs one last Concentrate to finish his ritual, and also needs an Attack to actually fire the weapon. 6 MP is the minimum he can spend for the Concentrate, putting him at skill 18. He spends 16 (14+2) MP total on Attack - 12 to attack at Skill+2, 2 to negate -2 for targeting the Vitals (leaving -1 left) and 2 to benefit from Acc (+4). Against a target 20 yards away (-10), that's an effective skill of 13. He rolls a 15 on the spell, successfully casting it, and an 11 on the attack. The enemy mage - who failed to notice Jack taking aim at him - is struck by that magically-enhanced bullet and collapses to the ground.