I wouldn't recommend it. For oxygen masks to work, there must be a perfect seal at all times. This is almsot impossible because the contours of your face will naturally change as you lose unconsciousness. In fact, 1 member in the dignitas study took almost 40min to CTB becuase her mask wasn't a proper fit while other particpants took ~2-4min. You're better off implementing a SCBA, SCUBA, EEBD, or the good ol' exitbag setup.