Gas is out.
If you can't regulate the dosage individually you run the grave risk of passengers dieing on you. Thats why they have an anesthethetist on hand during even routine surgery if the patient is "put to sleep."
The cockpit door and the surveilance cameras are a good idea. Why not do what El Al does; put armed men on board. Of course, that would mean that the airline would have to give up a couple of seats that could be sold to paying customers but it's better than losing a plane to say nothing of all those little bottles of cheap wine and packs of peanuts.

troll