Funnily enough I was looking at victory conditions for Quatre Bras only yesterday after following the link to the Junior General site that John D Salt posted in Whirlwind’s thread on Napoleonic Naval Rules. There is a scenario there which suggests that victory should depend on control of the three roads exiting the map board: the road east to Nivelles, the road north to Brussels and the road west to Namur. The French have to control at least two of these to win. If they control one it is a draw and if they control none it is an Anglo-Allied victory.
This seems quite sensible to me and I think would prevent the game from descending into a slugfest around the crossroads itself.