Absolutley not. First, remember that something designed to knock out an adult can kill a small child or baby.
Second, even with a double key system, there is still a chance of the wrong people getting their hands on the system, and I'm not talking about the bad guys either **cough ** tsa **cough**
With all the jets they scramble because some Okie from Muskogie is flying with someone who's *(looks around conspiratorially and whispers)* "Brown" for the first time, and he gets up to use the lav one time too many, I'm not sure I would want knockout gas on board either.
After all, there's no such thing as a OVERabundance of caution...