Unless I'm mistaken, there isn't a way to do this, and even if there was, it would be dangerous to do so due to what I consider a current flaw with how Forms handles submitted responses. We found that in the event that an option in a Choice field changes, say, B is edited to become C, every response that replied B turns into C. If Lookup were possible, and since those options can also change, I'd expect the same issue to occur.
