Ok so:
"Here's the thing - while on first read Hang Time requires you to be absolutely motionless, the specific thing(s) you cannot move are the points of contact that are providing the Hang Time. You could - I think - move any other limbs, so you could hang there with one hand and both knees (or just both knees!!) and draw a pistol and fire."
Yes you could do this according to how it's written.
"Or - and I'm just spitballin', here - you could hang by one foot and the opposite hand, move the other foot/hand combo upwards, lock them into place, and 'free' the first pair - and thus ascend using only that."
No you cannot do this, it specifically says you cannot in fact.
It's not Spider Climb, it's Hang Time.
How do we know this?
Because:
1). you only get a small die bonus to climb you do not get to run up a wall like spider man or gecko gloves. If it worked that way you would not get a die bonus, you would get to climb without falling. And there would be no focus on hanging and time limits for same, it would focus on climbing. Which it does not.
2). it only costs 0.25pp, wall running costs 0.5pp. A power that lets you free-climb anything without fear of falling and arrest your fall would be like 1pp or maybe even more, It's incredibly powerful.
So yeah, I understand why people want to make this work with Wall Running, I mean my toon is specifially built to be farkin Ninja with Traceless Walk, Wall Running, Freefall and Hang Time.
But no, it specifically says Hang Time NOT Spider Climb and the text, although a bit confusing, does specifically say MOTIONLESS as in NOT MOVING as in NOT CLIMBING and NOT RUNNING and NOT FALLING.
Nam my gymnastics dice pool is not that amazing, it's 9 dexterity + 1 balance augmenter + 5 skill.
He's very good but not superhuman ;-)