A question which has left me wondering for years now (literally a decade) has finally been answered by Sandy Glau, a former Continental Airlines Boeing 707 stewardess.
The 707-120s that Continental were flying originally (1959-era machines) did indeed come with the typical drop-down style oxygen mask system that you would find today on any modern airliner.
The usual scenario is that in an explosive decompression the duty of cabin crew was to ensure people had their masks on while keeping passengers calm at the same time- this would be no easy task in a jet descending rapidly with flight attendants probably finding it tricky to move with the rapid descent and associated g-forces of that descent. Nonetheless this was the procedure.
It would also be usual for cabin crew to receive further instruction from the flight deck at some point when the aircraft was at a safe altitutde for passengers to breathe without the use of masks.
Flight 11 of course lost its tail at some point during the descent and it is appreciable that cabin-cockpit communications would have been impossible.
