Perhaps frequent flyers could be issued with a card stating that they are willing and able to operate the overwing exits. They could be given a gold card if they agreed to attend a couple of hours instruction, given at most larger airports these days. If all the airlines agreed then they could be allocated these seats as they booked their tickets.
On the few occasions when there is no such card holder on board then the detailed breifing as is currently given by some airlines should suffice.