If you can get twenty people to do a WSG premade, you can get thirty to do AB. The problem is getting people interested in the first place. There is next to no interest, so it's never going to happen. The few people that do have it got it because it was/is bugged. Who knows what will happen when pandas come though?
Anyway, I made this thread about achievements that were possible, but not yet obtained by a F2P. I was wondering if there was any I had missed or was unaware of. The only ones that haven't been tested yet are One Hump or Two? by master looter. Ogri'la is not possible according to Wowhead, because the event has been removed. No one has bothered with Argent Crusade yet, and I have yet to kill Garr with a revered person to test if it's possible to get Hydraxian Waterlords to exalted either.
There's also multiples of looting gold, quests, quest reward gold, and tabards. The Infernal Ambassadors turned out to be incredibly easy. All I had to do was be grouped and phased as someone killed the daily quest mob. They didn't even have to kill them in a specific way for the most part because I, while phased, did not get hit by their special attacks, but did get credit for their kills, and so credit for the achievement.
Edit: For those interested, two of the Hallow's End achievements, concerning killing HH are grandfathered. I'm pretty sure that once the pandas come, Deathwing will stop killing people in random lowbie zones, so that will be grandfathered for low levels too.