[Ciaphas Cain]

There have been cases of trans-PACIFIC stowaways riding in unpressurized baggage compartments. But they had to figure out how to sneak aboard a LOT of insulation, thick blankets etc. Otherwise one is almost certain to freeze to death.

It's extremely risky. Few survive. But it can be done.

[Cobra64]

You need O2 over 15k. Blankets don’t help with that.

[CurlyDave]

There have been cases of trans-PACIFIC stowaways riding in unpressurized baggage compartments...

The baggage compartment is pressurized.

Look at the airplane. It is a round cylinder. Round things hold pressure well.

The floor of the passenger compartment/ceiling of baggage area is flat. Flat surfaces do not hold pressure well.

The entire cylinder is pressurized.