Dunno if it's worth the trouble, or if it truly solves the problem, but you could make it so if you fail to stab a foreign enemy noble (your realm is at war with it) in your own realm instead of being captured you get justice delivered directly by the injured party, taking a /very/ severe beating that leaves you almost dead (critically wounded). If you want you can even have it kill you should you be banned from the other realm.
The problem is that it adds unnecessary complexity with the special rules. I hardly think it can be exploited, since if you fail you get out of service for a good time.
