One interesting aspect is the fact that airliners are built to specs for a given pressure differential between the cabin and the environment. At 40000 feet, the cabin pressure is about 75 kPa, while the outside pressure is about 19 kPa, which rounds to a 60 kPa difference. You could therefore, in theory, pressurize the cabin to 60 kPa, with a 25/75 oxygen/nitrogen mix to have a partial O2 pressure of 15 kPa, equivalent to what is on the airliners at cruise altitude.
Then you'd need to apply some type of additional thermal protection I'd think, as the bare aluminium skin might radiate a little too much heat to space... On the other hand it might be a blessing, relieving the need for fragile radiators to be retrofitted if you dump the heat in the structure and let the wings play that part...