I'm not a football referee, but if the rule is he cannot be hit until after he has possession of the football, I think it would end the "blow ups" entirely. All defensive players would have to pull up before hitting the returner to make sure he actually has the ball, then makes a move. Even when caught on the run, this should work. We have targeting rules for defenseless receivers, why not punt returners?
Just spit-balling. I think it's workable--at least as workable as the current system--and safer.