The Ravenloft setting (Sword and Sorcery) has rules for this. They do it with D10 Fragments, and 2D10 Alter Egos. Age, sex, and race and randomly determined. Whenever said character has to make a will save, they are forced to make an additional will save with a DC set to the DC of the Madness save that causes the mental "problems". Upon resting, the character must also make a DC 15+WIS or randomly switch. Dunno if that helps in anyways, but you might be able to integrate it into what your looking for