I suppose they could do it in a general way. Say a percentage change if the opposing general is defeated. Probably would take some scripting to record the death of the enemy general, if that is all possible.
There are conditions in the EDCT that allow to track if the general was in combat and the number of kills. This is a part of a trigger in the Deus Lo Vult mod.
Code:
;------------------------------------------
Trigger battle3Chivalry_defensivewin
WhenToTest PostBattle
Condition not WasAttacker
and IsGeneral
and BattleOdds < 0.5
and BattleOdds > 0.05
and GeneralFoughtInCombat
and GeneralNumKillsInBattle > 8
and not Trait BattleDread > 0
Affects BattleChivalry 1 Chance 85
Or maybe they could have this trait have a percentage if you manage to capture the general and decide to execute him. Maybe not the same thing that you are looking for but it could be an compromise.
But then again, I really don't know what or if any of this is possible. It's just all conjecture and theory on my part.