VinRouge wrote;
What if they were already on the flight deck?
Still very iffy methinks. As soon as the flight crew reached for their quick-donning masks, so would the hijacker - for every flight deck jump seat there is a corresponding mask, and the pilots would not be able to disable them in-flight.
Plus, to depressurise the aircraft, you would either have to turn off the aircon packs or engine bleed valves (slow decompression), or deselect the automated system and manually drive the outflow valve open (depressurise as fast as you like).
Whilst I fly 767s and not 777s, so the details are likely a little different, but the basic system architecture would be pretty much the same. Playing with the aircon/pressurisation system on the overhead panel is not going to go unnoticed by the highjacker who would be watching the pilots like a hawk.
Any triple 7 chaps reading please do confirm or correct.