Just more of a spontaneous idea than a real suggestion, but how about a script, which sets the chance of capturing enemy lords to 100% when the player knocks him out himself?
I know of Robert the Bruce having fought a duel against an English knight, Henry de Bohun, before the battle of Bannockburn started. Although this was a rather seldom occurance, it would be nice to make the players actually seek the knightly duel with the enemy lord, instead of simply letting the cavalry charge by pressing [3]-[F1]-[F3].
So it's not hostorically accurate, but it would be a nice feature for single player, imho.