I had a couple of thoughts on this:
Potassium Sorbate is effective up to a pH of 6.5. Would it make sense to increase the pH of my mixture to, say, 7 using a a base, and adjust the pH to taste after fermentation? How would this affect the yeast?
A starter with just sugar and water seems to ferment, so the yeast will probably be okay. It might make the mixture more prone to infection, though. If the fermentation gets going well enough the yeast might take care of that.
Well, it's a better idea than just tossing the whole thing. I'll look into it sometime today.