The main question Im answering is that it IS possible to detect ionizing organic gases by existing everyday technology.
I don't think the question was ever whether or not the technology to detect exists. That, IMO, was never in doubt.
The question that was asked was:
Are you saying it would be possible to cover or otherwise disable the gas alarms in a plane's restroom?
That question is derived from an assumption that the existing technology was/is present.
After recent events I suspect that sensors will be installed throughout the passenger compartment in the near future like you mention.