The pole climbing strap would work. I just think it would be less than safe because it can move or slide. During this going on 2 and a half year overhaul I have been up several masts many many times and even in port the boat can get rocking quite a bit and that motion is really exaggerated at the top.
I would think about a safety harness around your torso with a short tether that could be attached to a hard point at the top. You could still lean against it for hands free working but you would never fall or slip any farther than the tether let you.