I know when I've flown CO they would board coach pax who are waitlisted, and then seemed to discretely pull you from your seat and move you up front if you got an upgrade.

UA doesn't do this, so if you are waiting for an upgrade and it falls through you are last to board with the same problems of lack of overhead space etc.

I think your suggestion is a good one but I can't see UA doing it although other airlines might (and they all should)