That won't reasonably approximate a Grand Melee, because in BattleMaster battles there's always exactly two sides.
If you get 6 individual nobles from different realms into a region and have them all set to murderous, you'll have one of three outcomes:
1) The game doesn't find a defender. No battle takes place.
2) The game recognizes one of the nobles as either owning the region or controlling the battlefield, and designates them the defender. They fight alone against all the attackers.
3) The game selects a defender as in #2, but they have one or more allies present, who join in defending; all others attack them.
