Larian has already expanded the UI so that you have room for six character portraits. It would't be dificult to make a six man party available for the players. Those that want a four man party need not take any more, but we who want a party of six would be free to do so.
It certainly wouldnt ... But its even easier to simply say "we are counting with that many people will use 6member party mod, so we counted with it when we created our UI ... but that is all support you get from us" ... wich is basicaly what they did.
Any problems related to that mod will then follow the mod and not them.
In the words of the senior NCO instructor at cadet battalion: “If you ain’t cheating you ain’t trying. And if you got caught you didn’t try hard enough!”