Not certain we could ever code and/or enforce, but I was reading about this as part of the Deadlands system. Thought I'd share.
Hindrances (Reward for Roleplaying)
Marshals are also encouraged to reward players with instantaneous rewards for good role-playing. During character creation players can choose "Hindrances" (disadvantages) such as Big Britches, Bloodthirsty, or Big Mouth. When a player role-plays this Hindrances well, the Marshal can reward them instantly with a Fate Chip. This has the effect of promoting and encouraging role-playing.
Found: http://en.wikipedia.org/wiki/Deadlands
Would be interesting if you could attach or roll a list of these that were available for Imms
Edit - Note: hindrances, not fate chips ;)