But above 40k they would need pressure breathing and I don't believe that is available.
Above 34K, IIRC. Max altitude with pressure breathing depends on somewhat flexible physiological constraints, but B-52 pressure breathing was limited in the Dash-1 to 42K.
And don't overlook the non-hypoxic effects of decompression: having your internal gases suddenly try to triple in volume, for instance, could prove... distracting.