I can't see that working. If you start to have to check who can carry on and who can't, you will show boarding down even more. It remains to be seen how United will deal with their new basic economy. It would be easy if they actually monitored boarding, but they don't. Flew first class the other day. Among the first to board (or I thought) and there was already 20 people sitting in coach (and they had only called pre-board, premium and "those who needed special assistance"). They have to get a system where the scanners reject people who have not been called, but that won't happen. Maybe "shame" the people who try to board before their zone is called!