We've been putting rats in skinner boxes for a lot longer than we've been subjecting human gamers to them. I'd be more worried about the health effects of all that sugar water.
Giving them slightly acidic water off-rig and normal water while playing is another option - not the best either. I opted for sugar in the end, because they didn't spend much time on the setup; but this should be reconsidered if they on-rig for hours daily.
