Keep in mind that the oxygen permeability of a glass carboy is not zero. Sure, the glass is not permeable but the stopper, whether it be a gum rubber stopper or a vinyl carboy cap is permeable.
I bet if you were to get a few belts and wrap them around the carboy you could use it as a keg. The belts will help prevent it from warping and exploding they will also help in maintaining pressure.
I'd keep them hooked up to a regulated CO2 tank, just like you would a corny keg.
For the record, I bought 3 Better Bottle 6 gallon carboys on the weekend.
I've decided I will use my cornys for carbonation and storage because 4 of them fit in the lower part of my fridge. I'll bottle or consume whatever beer/wine comes out of the BB carboys that won't fit in a corny keg. I guess its nice to have a little bit of bottled product on hand.