I found out from Tascam that the Accessory2 port on the 238 (and all later Tascam Multitrack Tape Machines) actually contains two comms circuits on the 15 Pin DIN. Some of the pins are allocated to the 'controlling circuit' which deals with the capstan control for sync purposes. The other pins are for what's called a 'communication circuit' which is a standard RS-232C serial port and that's the port which does all the machine remote control. I believe that is the part of the ACCESSORY2 port used by the MTS-1000 MIDIIZER for remote control of Tascam Tape Machines.
So far Tascam have found me a cope of the CD Controller protocol which provides a lot of useful information on the serial port but NOT the correct codes for tape machine control.
I have asked them to look again but if anyone reading this has a copy of the serial control protocol, possibly as part of a Tascam Tape Machine Service Manual, or perhaps within the MTS-1000 MIDIIZER documentation, then please let me know.