The FL310 limitation is based on the ability of one a/c pack to pressurise the cabin. It assumes the pressure hull and pressurisation system is intact and fully functional. This scenario assumes the pressure hull, or pressurisation system, will become compromised, yet there is no suitable, approved back-up oxygen system for the flight deck crew.
Assume the worst case, as pilots are trained to do, which is a rapid decompression to FL310 in about 10 or so seconds. The pilot with the makeshift oxygen system will hopefully mask up. This takes a finite time using a system that is not “quick donning”. Even if he masks up in 10 seconds after the decompression the non-protected pilot has now used up a considerable portion of his time of useful consciousness, possibly all of it.
“A rapid decompression time (less than 20s) to a cabin altitude above 30,000 ft will produce a significant impairment of performance in those breathing air even if 100% oxygen is delivered to the respiratory tract immediately the decompression commences."[Aviation Medicine, 2nd Ed, Ernsting and King. My italics].
Based on this, the unprotected pilot will suffer some impairment of mental function. If, during the descent, the flying pilot with the makeshift oxygen system takes it off to share it with his colleague, then there is a real risk of two mentally impaired pilots carrying out an emergency descent and subsequent recovery and diversion.
A few more points. The gauge says the oxygen bottle is empty but you test it. You think you can feel oxygen being delivered. Do you really trust that system? Remember, if it’s really not your day your life may depend on it.
Flight plans in pressurised aircraft should take into account the occurrence of a depressurisation and subsequent flight at FL100. You should have enough fuel to divert somewhere at FL100, otherwise somebody’s going to be in big trouble if the authorities find out.
Why risk it? If walk around sets were suitable for this purpose then wouldn’t it be clearly stated in the MEL that they could be used in this way?
The flight crews’ lives are worth so much more than the simple A to B journey they were originally tasked to do. Their passengers’ safety and welfare is certainly worth more.
Descend out of hypoxia territory.
Divert if necessary.
Pour yourselves a well deserved drink for making a good, sensible, although perhaps boring decision.
You know it makes sense.