Roughly yeah. Devil’s in the details as usual I guess.
Good point about rewinding on MIDI Start, although MMC Rewind is only the opposite of Fast Forward. So it’d need to be MMC GOTO 0 instead to go to start, GOTO 0 being equivalent of MIDI SPP 0.
The far more straightforward part would be arbitrary Note on → MMC op transformations which don’t need wacky semantic translations: all the MMC ops that don’t take complicated arguments, ie Sub-ID#2 from 01 to 0D.
Just as an example, Mackie control messages are something like:
- MIDI Note on A#6: Play
- MIDI Note on A6: Stop
- MIDI Note on G#6: Fast forward
- MIDI Note on G6: Rewind
- MIDI Note on G5: Record
…with a straightforward mapping to MMC counterparts, but varying dialects of Mackie/HUI exist so these shouldn’t be hardcoded, just Note on → something transformations (and optimally, vice versa).
Just to get this straight, I’m not trying to convince you these are Highly Important Must Have Things (they would be useful to me, but then my setup is probably pretty far on the weird side 
). I mainly wanted to point out in the context of this topic that there are truly generic SysEx messages with meaningful transformations to and from, ones that competition cannot do (well, BomeBox probably does, but only through separately sold OSX/Win editor software)
I’ll be happy to dig out and think/discuss further details if you decide to actually implement some of this of course.