IIRC, the oxy generators are only good for 15 mins, right? So hanging about at high alt aint an option if you want the pax to survive.
A private jet Captain visited us recently and told us if his jet (think it was a G5) detects loss of pressure above FL400, the autopilot will deviate and descend after a few seconds, assuming the crew are unconscious (think he said to FL120), and bang out an ACARS/CPDLC message to ATC (if equipped). Not sure how that integrates with TCAS/GPWS etc....