Slorkuz weasels his way out of a real answer by saying that this is already possible in game: places like the Gurubashi Arena do let whole groups of people fight each other however they like. He also says that Blizzard has no plans to add any features like this, but he's slightly wrong -- we did get told at Blizzcon that they were going to do more along the lines of Guild vs. Guild combat, if only in the existing BGs. This idea, along with guild housing, is still in the works somewhere, at least.
Of course, with any player interaction feature like this, there is always the opportunity for exploitation -- some players have probably refused enough duel invites in their day without having to worry about any more, and whenever you have someone "speaking" (as in, accepting or rejecting a duel) for a group in terms of PvP, you can run into trouble. But for those who really would like to know whether their guild could beat another guild, or just which raid is better than another one, a feature like this would definitely be welcomed.