I'd probably stay with: 0hp? If you decide so, the opponent is disarmed and at sword point instead of killed (or unconscious). Simplest solution.
What I don't really get is that the guy says he's a "simulationist". Does he know that disarming an opponent of his weapon, with which he's trying to kill you with, while you're doing a very precise set of maneuvers in order not to harm him, is WAY MORE difficult than outright try to kill him?
Famous "disarmers" I can think of in literature and movie are all at least paragon level, from Zorro to Errol Flynn. Everyone under that level of skill should have a bad time triyng the stunt.
Any alternate system for disarming, should be outright more difficult than the built in combat system.
If I *really* had to invent on a spot a system I'd go something like this:
- Every monster has a separate pool of "disarm points" equal to his bloodied value.
- When you try to disarm an opponent, you deduct damage from its DP. All special conditions and typed damage are not taken in consideration.
- You must be using a melee weapon (at -2 to hit) or a ranged weapon (at -4 to hit).
- DP are fully "healed" when nobody attacks the monster for a full round.
- When DP go to 0, the monster loses its weapon.
I know this process seems long and difficult to perform, but this is how I think is should be in a "simulationist" scenario. Of course some of the penalties could be offset by feats. Becoming a good "disarmer" should involve devoting most of your feats.
I haven't thought how much the penalty for this should be, but since the process for disarming is difficult, it should be significant.
But as I said, I content myself with the general system. Though some of the powers proposed here are nice. But I would not make them anything less than encounter or even daily. Disarming is too much a signature move to be an at-will.