That's an interesting concept. The fermentation of 1 pound of glucose yields about 1.6 cf of CO2. I'd have to have to crunch those numbers to see if the benefit is worth giving up green house space.

The other challenge would be temperature stability. We make kit wine, and the instructions usually say that fermentation should take place between 68-77F. If you had a green house that stayed below 68, I suppose you could add some supplemently heat to the fermentation vessel and manage it that way.