ok, since we're all enthused about this idea and we're talking about sweet code, how about this one:
Max number of players attacking player X = 6.
So, you run up to X and attack, 5 of your buddies do too, but the 7th guy has to wait till one of you disengages or finds a new target.
I was originally thinking 2 or 3, but that is pretty low, many profs can actually tank 2-3 players quite easily, notably doc, enf for a bit, advy and soldier.
But this would really, REALLY feed the paper/rock/scissors thing, and it would really put a hamper on gimps coming to Tara. It's also highly sploitable (i.e. roll 6 opposing faction toons, lock target and roll in unopposed).