I went back to a broken savegame and did the following; none of these stopped the crash on rest other than control-R healing.
* Kicked everyone out of the party.
* Burned every spell and innate ability on my bard except Slayer form and removed all spell memorization. Only spell association is a ghost armor icon in quick cast.
* Removed every item except familiar and green ioun stone.
* Changed from strategems ease of use AI to no AI, with AI on/off.
* Removing the green ioun stone fixes it, probably just because it takes me back to full HP. Verified this by taking off the stone and then using a fireball scroll to damage myself; resting then locks up. Killing my familiar doesn't change anything whether I heal up or not.
Finally, the acid test: no familiar, no stone, control-R to heal, successfully rest. Use fireball wand to damage myself. Rest - and it locks up. Ah ha!
* Resting like normal to get back into a good state. Verify fireball damage from 98 -> 76 HP causes the freeze.
* Use the console to bump myself to level 32. Use fireball to damage from 130 -> 120 HP. Resting no longer causes the freeze.
So, possibilities:
1. Something about lower levels causes it; it repros at level 16 for my blade.
2. Something about maximum hit points below a certain threshold causes it. Maybe it does fractional heals or something at certain max hit point totals and can't handle it?
3. Something about slayer form causes it. I tried resting with an earlier with slayer form and no sign of freezing, though.
4. Something something mystery aura state that only leveling cures.