Rather than ATC taking over the plane, which seems like a non-trivial exercise, it might be a lot easier to allow them to override the door 'lock' mode in the c

Rather than ATC taking over the plane, which seems like a non-trivial exercise, it might be a lot easier to allow them to override the door 'lock' mode in the cockpit (which ignores the keypad code for door entry) so that in this case the pilot/others outside could have asked ATC for it and got in. This means that there isn't someone outside the cockpit on the plane who has 'root' access (so nobody can be threatened for it). Even then, there are considerations like not allowing the cockpit to kill all com links / power so that people outside the cockpit are still able to communicate with the ground to get the root pwd, and as mentioned elsewhere the person inside the cockpit could change their strategy to make recovery of control difficult or impossible even in this case (EDIT: e.g. throwing the plane about). Also, you'd probably want >1 ATC person to authorise to reduce the chance of the door being unlocked by someone at ATC in cahoots with hijackers.