A thought;
Simply move the cockpit door rearwards so that the forward toilet becomes part of the flight deck, (sorry passengers - you will have to use the rear toilets). That way, no pilot can be locked out of the flight deck while going to the loo.
The only time that either pilot would then need to go through the locked door would be to board or de-plane at each end of their flying duty.
The controls and operation of the locked cockpit door by the pilots would be just the same as it is now, but the door would be moved about a meter back.