Some thoughts as a technical diver, rather than a pilot.
Breathing the wrong gas has killed plenty of divers. Cylinder markings, colour, labels etc don't make any difference.
At the end of the day you need to know what you are breathing, that you have enough of it to get you to safety, and that the regulator and mask will deliver it when needed.
The only way you can be sure of this is to analyse the gas (few seconds work with very cheap oxygen analyser), check the cylinder pressure, and breathe the system to confirm it is working. Repeat *every* dive.
The same simple and quick checks would certainly work in the cockpit, and the atmosphere at crusing level is no more compatible with life than 50 metres of sea water.
CC