> sequential writes (i.e. log files), if a append-only file is written when it ends in the middle of a block, that block should be able to be modified rather than copied when additional data gets written
I believe sequential writes are already handled optimally by flash firmware. All writes to flash can be easily buffered in on-board RAM and merged into block-sized writes before hitting the flash. Of course, such RAM must be backed by a power supply (a capacitor), which will allow flushing RAM contents to flash in the event of power loss.