Correct, there's no published rule that would say "this is what the price should be for this service." In practice, you will likely get a free bag, but I can't point to anything on the website that would demonstrate that. It's just that I know how they generate bag tags -- they add your ANA segment temporarily to your UA reservation -- and I know what UA's computers will do in that situation. But they could change it tomorrow and there's nothing I can point to that says they can't.

There's no point in calling or tweeting. Nobody will be able to give you any sort of answer that would do you any good if you show up at CVG and they've made a software change. It's just, that's not very likely (but, admittedly, not impossible; just really improbable).