In theory, I think it's a great idea. In practice, there are three factors to consider before getting this off the ground.
1) Running a guild takes sustained time and effort, even if the guild is just a hangout for all alliance 29s. At some point issues come to bear that need addressing, so someone has to step up and take responsibility or everyone in the guild suffers as a whole. For example, when Hard was the "29 hangout" guild late last year, and the 4.0.1 patch turned sub rogues into ridiculously OP killing machines until 4.0.3 came out, a ton of rerolled sub rogues brought a lot of heat to Hard players as a whole. Finally, the GM kicked four of them out of the guild to show that he prioritized the good of the bracket over the will of some FOTM rollers. He could have chosen otherwise and told everyone that Hard was just a guildhall for the bracket (which it was, at the time). Either decision would have brought criticism. Make no mistake, putting everyone together in one guild means someone needs to lead, even for a "guildhall" guild. That's why this suggestion comes up every couple of months, but no one has acted on it yet.
2) The 29 bracket is in the middle of a major culture clash. If I may oversimplify, veterans from the tight-knit community of the mid-3.x-era are much more bracket-oriented, believing that regulating the bracket will bring the same tremendous success the bracket experienced last year. Pre-3.x veterans and rerolls believe the health of the bracket begins with the good of the individual, which likewise brought tremendous success for them in the past (if you intend to comment about this comparison, please start a new thread so that the original poster's thread doesn't get derailed). This polarizing culture clash has long since passed the bounds of respect and decency in 29s, and getting all the players on Windrunner under one roof is very unlikely until this clash settles down.
3) As cool as it would be to get alliance 29s under one roof, the (now working) ability to queue from different battlegroups and consistently get games weakens the appeal of moving to Windrunner for just a casual hangout. That leaves the current Windrunner population as the main people potentially interested in such a thing, and I honestly don't know how large a population that is anymore.
TL;DR version: An alliance "29 hangout" guild is a good idea, but promoting, building and maintaining such a thing will be anything but casual.