Thats a very good point. If there is any uncertainty, it is best to ask a Vet, and even then you get different opinions. Most mammels are also just as likely to become dibetic as a human so there are lots of good reasons to just use water. Even though rabbits are more sensitive then many animals, in an emergency situation, I would still choose to use it and risk the side effects. I've lived in the desert about half my life and seen many creatures die of exposure and dehydration, but it is of course, a pet owners choice to do what they think is best for their pet.
Incidentally, I know Gatorade is 6% sucrose and or fructose and at 7% the sugar starts to have a negative effect on digestion or block mineral absorption. I have heard of giving rabbits fruit juice and this is generally much higher in sugar, but that is mainly to give them a vitamin C boost.