The low-tech solution posted earlier on another thread is still relevant so I apologise only to those who have already seen it.
Pilot wishing to leave the flight deck calls the FAs and waits until one appears at the cockpit door, enters the code and is allowed into the flight-deck. Pilot leaves and door is locked behind him.
Pee and chat take place for a few minutes until the pilot enters the code and the FA looks through the peep hole and the seated pilot confirms via camera, if fitted, that it is indeed the pilot outside the door. Now the safe part, the FA opens the door by using the door handle!
Result, no stupid switch position will ever catch you out again.
Cost; zilch.