I occasionally do up a batch of homebrew, which I guess would count as a "yeast/sugar concoction." I leave it to ferment in the tent, releasing CO2 constantly, in 5-gallon batches. I just went through all my grow journals and crunched some numbers about a week ago, and all other things being equal (it's been by no means a properly controlled experiment), harvests with homebrew going have been 12% heavier (12% more production in dry weight per plant). But like I said, I've used different strains here and there, vegged some longer and some shorter, etc., so by no means controlled for other variables. My tent cranks out two of my favorite vices that way, and seems to help the production of the green one, so bonus for me. If anyone's going to give this a shot and doesn't know much about brewing, you have to brew an ale (higher temps) and you have to have some way to keep the brew from being spoiled by light (I use a thick t-shirt around the 5-gal carboy, rubberbanded at the neck).
Cheers...