AFAIK, the only data saved during powerfail is to flash - there wouldn't be enough power or time to spin up the disks.

Correct. The stuff saved to the scratch partition is done at the moment you perform the relevant action. For instance, toggling shuffle, or saving a bookmark, or selecting/inserting a song or playlist, those cause a scratch partition write at that moment. Then the current position within that songlist is saved into flash at the time of the next powerfail.