When we first got the 'K' we did have lots of crew entrance door problems in flight. Hence the agricultural reinforcement you see around the exterior of the door. The sytem 'cured ' most of these by removing the 'door locked 'microswitch and just leaving the 'door closed ' microswitch in place. I have commented on an earlier post my thoughts on this engineering masterstroke but if anyone needs a recap I will do so.
As for chaining the door , the weak link was on the door itself where the chain would need to go. A mot point whether it would have retained a door bent on doing a runner.