Pressure breathing equipment is not standard in airliners. It would have had to get on there somehow, and not in a carry on: An O2 tank of sufficient size would cause all sorts of alarms, considering what happened to me and my spinlock life vest on my last transpacific flight! It's got a 20 gram CO2 cartridge in it which made the doozies lose their plots, knickers their twists and all such things.
Having said that, climbing to FL450, depressurising, waiting until you're almost out of O2 yourself (~30-45min?), then descending to a manageable FL295 on a low O2 flow to maximise range sounds like an awesome plan - but not a if you think about it in detail. Out of your 23x specimens in the back, there are at least 10 or so that will come back without permanent damage. Altitude tolerance is extremely individual. I don't think that's a risk I'd take.
I think I like my theory still the best: A spook sliced them open near where contact lost, that might have well resulted in the reported altitude excursion, crew and pax out etc, and subsequent fireball into the ocean. The PSR target then was the spook making its way out of the area, apparently not fully disabled at that stage.