Very Plausible
I haven't flown the -400 for 7 years, but it has---or had---an old-fashioned bottled-O2 system for crew and pax; none of this fancy chemically-generated stuff. I think it was two bottles, in the forward and aft cargo bays.
While O2 bottles, Halon bottles, and the like are sturdy (and steel), they can fail--especially if abused.
As a USAF student pilot many years ago, I still vividly recall a T-37 on the ramp at Williams AFB with the tail blown off--a new crew chief had tried to fill the bottle using high-pressure O2 instead of low-pressure. The bottle ruptured, and severed the fuselage aft of the engines.
Since other posters on this thread have discussed Quantas going towards outsourced maintenance, is it possible the bottles were ever over-serviced at some point, then bled-down? Or was it just random luck, or fatigue?
As to the UAL 747 that lost the door: the latches on the forward cargo door has something like 12 pins, visible through sight glasses on the outside (somebody help with the number). On that flight, the pins appeared to be closed, and all lights in the cockpit verified that. However, only a few pins were actually engaged. At FL240 when the pins let go, the upward inertia of the door ripped the hinge, and a dozen feet of skin above the hinge, off the airplane. The hole was huge, and that is why 4 people were sucked out to their deaths.
Some of them went into the number 3 engine. Other debris (I don't know if any of the people) FOD'd the number 4.
The only reason they made it back to HNL, with two out on one side, was because they were already at altitude.
As I remember it, for quite a while at UAL (it may have been when I was at Evergreen), you were required to pull all the circuit breakers for the motor system that locked and unlocked the pins, to make sure it wasn't an un-commanded event, before they had found that the door could be improperly latched, yet the lights would still go out. I think Boeing was found at fault on that one. This may have only been on the -200, though.
Rocketman:
I do have a pretty good imagination. Engineering is imagination paired with science.