You don't even have to lock them, do you? For safety reasons, passengers need to be seated and buckled in during takeoff. So you can just verify everyone is se

You don't even have to lock them, do you? For safety reasons, passengers need to be seated and buckled in during takeoff. So you can just verify everyone is seated, then after that check that the lavatories are empty. If a lavatory is occupied, make that person take their seat. Now you've verified there are enough seats for everyone.