Wiggy is quite right when he mentions changing the bang-seat bottle from 10k to 5km. When we flew over Turkey and Iran we always joked with the pilots that we would be OK and would find them by looking for the hole in the snow. The rear crew, free-fall, barastats were set to 12.5k, just enough for a HALO drop <g>.
One thing we never dressed for was a high altitude bailout. It was just something we did not want to consider. The chances of the jet breaking up was beyond consideration. At 40k to 56k we would have been into the pressure breathing regime as well as all the discomfort of the cold. The 2 mile drop would, I guess, have taken something like a minute or more and the temperature would still have been -56 when the pressure breathing came off. Then we would have had a further 4 mile or 2 minutes until the parachute opened.
Warming up at 12k would have been a slow process.
It did happen. I believe a Canberra disintegrated when its 8 inch flare ignited under the tail. The two-man crew, I understand, did not so much eject from the jet as the jet disintegrated around them. This was in the late 50s or early 60s.