ExAscoteer,
did 35000 ft free fall trials in the UK when I was at JATE. Very cold once the ramp and door was opened. As the a/c system was not cleared for this height we had a special oxy supply fitted at 245 with the oxy tubes routed to the flight deck. We had to pre-oxygenate for an hour before getting airborne. Smudge when I did high level para I always carried an extra Mk 4 portable oxy bottle with a locally made harness. Whenever possible on these trials we always carried a safety ALM whose sole job was to see the 'big picture' whilst the other ALM carried out the SOP drills. At those heights any small problem could escalate into a major emergency very very quickly.