realistically they'd be dead of hypothermia before reaching hokkaido
Not if they share body heat they won't.
Also the cargo hold has a small level of temperature control, enough to keep it from getting to actual freezing.
edit: it wouldn't be a particularly comfortable trip, but not lethal.
The "stowaways" you read about are usually in landing gear wheel because without a time-stop watch there is zero percent chance you are getting into the cargo hold as any time the door is open its going to be watched. People sneak into landing gear wheel wells because you might be able to slip in while no one is watching. And those will be zero insulation and zero pressurization. Additionally they are usually boarded by people in warmer regions meaning they are not dressed for high altitude conditions.