I'm thinking if the surface temperature were hundreds of degrees below, and the air were thick enough for conduction, that the suit would have to be a vacuum-insulation type of thing anyway?
No. Ambient-pressure-airtight is much easier than vacuum-airtight.
A drysuit for scuba diving is ambient-pressure airtight, basically just a neoprene coverall with sealed seams. It's easy to move around in.
But take away the external pressure, and you're in the middle of a 15 psi balloon,w which wants to be a sphere, not human-shaped. It becomes difficult to bend your limbs and fingers. Rotating a shoulder, neck, or wrist causes the suit to twist in like a party balloon. Anywhere the cross-section is not circular, it wants to be more circular.
You either need a hard shell suit, a tolerance for this difficulty, or a lot of engineering.
