MMC/SD card interface - any access to wear levelling info? (counters, etc)
I don't know whether particular SD cards expose wear-leveling information, but for the most part I would suggest that your desire to avoid erasing blocks that hold FF's is misplaced.  Even if a virtual disk block happens to only contain FF's, it will almost certainly contain other addressing information and error-correction data which will have to be rewritten if any changes are made to the block, regardless of its previous content.

I believe SD card manufacturers are free to select their own algorithms for deciding when to rewrite blocks which haven't been accessed for awhile, and for ensuring data integrity in the event of power failure.  Consequently, I don't know of any particular method of ensuring that an SD card won't get corrupted if the power fails during a write.