Tangential thinking out loud: this makes me think of a sort of interleaving or striping mechanism that tries to leave a small proportion of every track empty, such that ideal power loss flush scenarios would involve simply waiting for the disk to spin around to the empty/reserved area in the current track. On drives that aren't completely full, it's probably statistically reasonable that for any given track position there's going to be a track with some reserved space very close by, such that the amount of movement/power needed to seek there is smaller.

Of course, this approach describes a completely inverted complexity scenario in terms of sector remapping management, with the size of the associated tables probably being orders of magnitude larger. :<

Now I wonder how much power is needed for flash writes. The chances are an optimal-and-viable strategy would probably involve a bit of multichannel flash on the controller (and some FEC because why not).

Oooh... I just realized things'll get interesting if the non-volatile RAM thing moves beyond the vaporware stage before HDDs become irrelevant. Last-millimeter write caching will basically cease to be a concern.

But thinking about the problem slightly more laterally, I don't understand why nobody's made inline SATA adapters with RAM, batteries and some flash in them. If they intercept all writes they can remember what blocks made it to the disk, then flush anything in the flash at next power on. Surely this could be made both solidly/efficiently and cheaply...?