I'm wondering why you think she'll stop nursing if she gets sick enough, and how you'll get peialyte or anything else down if she won't take BM? A cup or syringe? If you can get anything into her orally, it should be BM. That's the best tolerated and best for her. See kellymom.com for more on that.
As to the suggestion that sugar inhibits the immune response, I've never heard of such a thing, and it doesn't really make sense to me that it would. Breastmilk has sugar in it. And we always give dextrose (a sugar) in the IV if we have a patient who is unable to tolerate oral food and hydration because people need sugars when they are sick so they don't start catabolizing their muscle mass for fuel. Fighting illness takes a lot of energy!!
If you're worried she won't nurse at the breast, pump some BM so that you can keep her hydrated via a bottle or syringe or whatever. That way, she'll get the fluids, the 'lytes, the energy, the antibodies, the white blood cells, etc. That she needs to help fight that bug.
Good luck, and hugs to you--sick babies are no picnic.
--Denisemarried since 2000, mom since 2007