The CD-500 protocol is very similar to the CD-400, and includes jog and shuttle as commands 15 & 16 respectively.? Both of these can be forward or reverse.
The CD-400 protocol I found is for v.1.03, and the latest firmware is v.1.11.? Maybe Tascam added jog & shuttle between 1.03 & 1.11. It's worth a few minutes to test them and find out if they are implemented.
See page 6-8:?
Good luck!
Marty
Martin Philip