The A-14 system, so I thought, is not a true pressure-demand type required at greater altitudes, but is a diluter-demand pressure regulated system with manual pressure valving available(?) Not to be confused with "pulse-demand" EDS. If needed I have a diluter-demand mask with microphone that can be loaned to Keith for his adventure.

While a cup of blood (or water for that matter) will boil at that altitude, circulating blood in a semi-sealed body will likely not (look up "Myth, boiling blood"). Mouth saliva would freeze first. Amazingly, experts say the astronaut in "2001 a Space Odyssey" caught in open space could theoretically have survived his short exposure.

If there's one locally, take a "flight" in an altitude chamber to discover how you function under hypoxic conditions (or you can try breathing a mixture of N2 with air/O2 to get your blood sat to 90% and less).