AEA for high-level PR9 ops in latter days consisted of only the pressure jerkin and the g-suit. These were both fed by the output from the Mk 21 oxygen regulator which also fed the oxygen mask. The relatively high pressure feed to your mask was stopped from bursting your lungs by the equal pressure in the jerkin and g-suit! This would give protection to above 50,000ft.
AMTW training for the PR9 included a rapid decompression in the one-man chamber at Farnborough; 25,000ft up to 56,000ft! Not exactly comfortable but it would certainly allow you to stay conscious and keep flying the aircraft to initiate a descent. The latter was flown with full airbrake and open Flare Doors. Quite demanding handling in terms of pitch and very easy to over control. If you got too fast the aircraft's significant pitch-up was greater than any nose-down from the elevator which would clearly somewhat abate the descent rate.