The problem isn't the stays.
There are actually plenty of sufficiently long chainstays available. Many hybrid, comfort, touring, and tandem bikes (just to mention a few) use chainstays as long as any 29er - long stays = more give = more comfort. At least for some people. Truetemper alone makes 3 or 4 models of chainstay that will work with a 29er. The problem, as several people have noted, is clearance, especially front derailleur clearance. With a 2.1" tire on one of my bikes (445mm chainstays, for the sake of argument), the front derailleur has less than 3/8" of clearance from the tire. A 2.3" is going to practically rub. A 2.5"? No way. Maybe for a SS if you were willing to push everything outboard on the driveside and have a pretty weird chainline. Not as much of an issue on the front, of couse, since many forks have adequate clearance for pretty big tires.
The other issue is that, if you want truly huge tires AND big wheels, a 3" tire on a 26" rim will give you an effective wheel size around 28". Not quite a 29er, but close enough, I'd think. So the impetus isn't really there - the FR/DH market already HAS wheels bigger than 26", because they all run huge tires.
I'd rather see, in order of importance: a 29" UST tire, more good-quality (ie Stans-able) kevlar beaded knobbies, and more quality MTB-specific 700c rims.