"Any method of getting in from the passenger compartment would be vulnerable to coercion."
not if you have an air marshall with an AA12 shotgun full of beanbag rounds right there. There are very simple answers, the airlines are whiny bitches that claim they cant afford it, and that is the center of the problem.
3 crew on flight deck for all aircraft, Let the pilots rest and get good sleep, and2 very well armed angry air marshalls on every flight. Solves 100% of all "terrorisim" problems.
Sadly the people in charge of security that have zero clue as to what to do, and the airlines whiny executives disallow the correct answer.