See the "cross-feed valve check" thread - APUs which are hard to start at altitude are started and run constantly on ETOPS legs.
The BA 747 which lost all four engines in volcanic ash eventually had the cabin altitude warning horn go off, and the cabin masks drop. As the FO's Oxy mask wasn't correctly fitted (it came to pieces as he grabbed it, as I recall), the Captain increased the glide speed to descend to 10,000 feet (left with a decision about whether he wanted a non-hypoxic FO, or more gliding time, he chose the FO!
)