Hi Pace... re:
A master key hung around the neck of the exiting crew member which would override any blocking attempt by the remaining crew member in the cockpit would work with a proviso...
Still waiting for a response from a A320 Captain regarding whether this could work ?
Before seeking a solution, we need to clearly define the problem?
With this crash what is the "problem" we are trying to solve? The ill FO on the Flight Deck, or the Captain being locked out of the Flight Deck
as per design requirements?