OK, what's wrong with using ACV in a galvanized waterer? I know it's an acid, but it's a very MILD acid and that shouldn't cause much of a problem leaching out whatever nasties from the steel. If anything, it should inhibit bacterial and algal growth. My 5 gallon galvanized waterer is really old anyway (but not rusty), and I don't think there's much that could hurt it.
I guess I'm questioning this because I've heard it repeated all over the BYCF but nobody gives any really scientific rationale for only using ACV or meds or additives in anything but plastic waterers. Which, btw, are often formulated with PCB and PCPs and those WILL leach out with acid additives. The only thing really safe for additives is glass.