So how long will it be until...

Funky idea. Might be a good option if you're stuck with bling rear hubs to create front wheels for a rigid build. The Endomorph tires are about the same diameter or larger than a 29x2.2", special headsets are on the way, one could make it work quite well... But then, any rear tire, even a 2.8" Frankentire (bikesonsnow did it) would feel skinny in comparison.