Yes, most seats (ACES II, for me) do have a "bailout" bottle that provides oxygen to the normal O2 mask. It is activated by a green knob that is part of the harness. It is about 2 inches by 10 inches, as I remember. More than enough to get to the baro parachute activation altitude of around 10,000 + or - some tolerance.
Also, correct on the TAS problem, opening shock is very severe at altitude, all seats delay opening 'til about 10,000 MSL. Don't know about bailing out above high terrain, that was never discussed in Life Support class. Guess the assumption was you would be smart enough to open the chute manually. Memory is fading on the details.
My ejection gave me about two swings to treetops, but ejection seats are fascinating bits of technology.
Gf says.
Oops, just remembered the bottle is part of the harness, not the seat. The seat goes away from quickly.