Well, I made good progress over the weekend,
First I spent time with al the bits of documentation I have, and the pcb from the typewriter (it's very fine tracked, but single sided, easy to read).
Then I instrumented the PCB with some probe pins to Serial2, and brought them, 5v and Gnd out and refitted the keyboard.
Somewhat unsurprisingly I didn't see any signals at all on either pin, they go low->high at poweron and remain that way.
Having researched stuff I was totally unsurprised, there are no variants of this typewriter with a serial/centronics output, and this board/processor combo does not appear to support any external (non-debug) comms.
Given time constraints I'm going to put this aside now, if it was quick-n-easy I'd have gone further but at this point I'm into interpreting and responding to the 8x8 scan matrix of the keyboard. Quite doable but not on my time budget ;-)
Something earlier and similar is here; the author of this went further than me and has a arduino sketch that sends keystrokes to Brother typewriter of the same era that appears to have the same keyboard setup.
I have some great pictures of the pcb and keyboard track layouts, so I'll upload them up too for reference.
I'll stuff all the useful photos in an archive and upload it for reference.
Note: This is the *back* of the keyboard,the key positions are mirrored.