I suspect that many people would argue that it would be unethical for a sentient alien race to eat us (humans) because we’re sentient, too. The problem, of course, is that most animals are also sentient.
As for me, I went vegetarian several years ago for multiple reasons, one of which is the sentience argument. That’s my line of demarcation, at least for now. That said, I definitely would not declare my vegetarianism morally superior to anyone else’s eating habits for two reasons. Firstly, I’m not at all confident that I’m right about this issue. Secondly, I do believe that eating meat is perfectly natural (I’m just not sure that it’s moral).
At the end of the day, I’m way more concerned about the ethics of factory farming and the environment. Those are the main reasons I went vegetarian. The sentience argument was a factor, but it wasn’t the factor.
However, in my opinion, I do believe that if it’s ethical for us to eat other sentient beings simply because they are less intelligent than we are, then the only logical conclusion to the scenario you mentioned is that it would be indeed be totally ethical for an advanced alien race to eat us (as long as they treated well us until it was time to slaughter us). It’s a chilling conclusion, but it’s the only conclusion that’s consistent with the logic we use to defend eating meat.
Good piece, by the way! This is a very interesting subject to me (and an important one, at that).