Hickory: I've never tried this, but I would imagine something like a reverse regeneration effect, but with a fairly high level of saves vs the effect. The reason I mention the saves is because the options you would be going for would be 'per second', with an engine imposed minimum of 1, so if you had -1 HP per second your character wouldn't last long at all. Like I said, I've no idea if this would work in-game. Nice idea though.
-1 HP/second would be a serious limiting factor, but there are still ways to deal with it. Consider:
1. A character with 40 HP can survive 6 rounds of this; death won't occur until the 7th round. Remember that mages have incredibly good defensive spells to the point where they are usually not taking damage from enemy attacks.
2. The character's life can be extended with healing magic; this change has the side effect of making healing magic more important, as it is more than just for surviving enemy attacks.
3. Regeneration effects can be used to (partially) counteract the negative effects. The Regeneration spell is strong enough to give the character a net gain of HP (it heals 3 per second, so the character gets net 2 per second while active).
4. Once the battle is over, the robes can be removed with the "Remove Curse" spell (the one that nobody uses to the point where Spell Revisions replaced it with a new spell).
On the other hand, I believe per round regeneration exists, so one could try setting the regen to -1/-2 per round if a weaker effect is desired.
Of course, this is assuming that this method actually works.
