If you use a gas there are a couple of problems:
a) it would have to be EXTREMELY quick - or you have a dead flight crew.
b) if someone bursts in and you set off the gas then you have one sleeping flight crew as well, they wouldnt have time to put on their masks (which would presumably be provided), if a) holds true.
Think stun guns are the best idea I have seen put forward on here.
Julian.