I'm going to stick by my comment about adding a adv/dis to BRP would be easy. I've seen quite a few of them, and they pretty much boil down to: take so many points or levels of a disadvantage, and add so many points to your character points. Take so many points or levels of an advantage, subtract so many points from your character points. It's always about evening out the characters with the rest of the group, while trying to add a new dimension to the role playing.
Personally, I don't care for them unless they are significant. Every person I've played with always played their characters with good adv/dis, and the gameplay always took the way they played their characters into consideration. If there were reoccurring patterns of behavior in the characters, the GM would just have the player mark it down as a trait. And the GM would throw in "temptations" and whatnot to get the player to use that trait.