Pace. I am not an A320 captain but will give you a response to your Master Key idea.
No it will not work.
Actually its along the right lines. I work in the access control business and we had this discussion over lunch today.
Bio-metric ID could be used in conjunction with a PIN to access the door. So like Pace said you have they key (your iris or fingerprint), and then a PIN. This PIN/Key arrangement is quite common in high security buildings. You would have to remove the ability to prevent valid "keyholders" from being denied access from inside the cockpit, which is currently the case.
In a duress situation the PIN would be changed slightly by the keyholder to trigger a "duress alarm" whereby access is still given. The "duress alarm" could then be used to trigger a distress signal, possibly a squark 7500, to allow a controller to see that someone has gained access to that cockpit under duress.
Certainly the knee-jerk reaction we are currently seeing could present more problems than it solves.