Summary: Historically tournaments were typically held as Melees which simulated battles. This could make an interesting new option for tournaments.
Details: to keep it simple the system would work out exactly like normal tournament combat, only using the leadership skill. If we wanted to get fancy there could be a combination of leadership and sword fighting. In the game fluff it could claim that each noble is commanding a squad of lesser gentry.
Benefits: Gives players that have built up their leadership skill a chance to show it off and compete just like players that built up sword fighting or jousting.
Possible Exploits: It might make hosting a tournament to distract from a war more enticing since an event like this might attract Generals or Kings.