Fergus,
I like your system, but I have an idea that I think might be an
improvement. Though you allow for O or D, you only allow 4 relative
directions (F, B, L, and R). Would it be better to have 8 relative
directions? Then a Knight would be (O#) where # is whatever symbol means
turn 45 degrees either way, rather than the current (OFT). Though it
doesn't matter much for the Knight, I think it would make the Mao
clearer.
This would also be adaptable to hex board by defining 6 relative
directions.