Churchill's Egg
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Looking at the question from the other direction - according to Wiki -
The York was not pressurised either, but a lot of work was put into 'Churchill's Egg' - a pressurised pod to go into his personal transport - Ascalon' the fourth prototype.
It was decided not to proceed, but the pod was then considered for fitting into an RAF C-54B - presumably for Churchill's use again. Again, not proceeded with - which suggests even the C-54 was not pressurised.
It's probably a matter of how do you seal all those gaps etc, to keep the air in.
Now - if you'd asked about the pressurised Wellington bomber, or how they shared out the oxygen supplies over the Hump ....
LFH
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