So it seems I am alone on here
I think it's pretty much released now, DIYAUTOTUNE have the GPIO board + kits availabel now for a couple of types of transmission, but it will not be restricted to the just the GM boxes, for instance there are people already running the ZF HP24 with one
From what I can read there's two ways to go about it, one with MS2 controlling the engine, and that provides information to the over the CAN bus to the GPIO board such as engine speed, TPS, MAP etc, and then the GPIO board works it all out and switches on/off various solenoids on the auto box.
The second way is making it a standalone gearbox controller, feed it a MAP or TPS signal and an engine RPM and it calculates everything as to what gear it should be in and whether lockup should be engaged etc, spitting it out to the solenoids.
We'd be going the standalone route, as the Lex V8 is currently running MS1v3 and not likely to change
When I find a bit more on this I'll update it, just trying to find it is quite hard at the moment...