This week I successfully turned off the Airbag light on my 1995 164Q - so I'll write up a few notes.I used Wilco's Windows software downloaded from the start of this thread.
I bought the UK Lonelec USB-to-OBD box/cable mentioned above and the 3pin-to-OBD adapter from somewhere else. I also previously tried a cheap USB-OBD cable from Ebay but it has the CM340 chip, not FTDi and I couldn't make it work at all. I should have gone straight to Lonelec for both (ordered online in California Sunday, delivered Thursday)
The ECU 3-pin diagnostics connector on a 1995 164Q is behind the carpet, sort of above a passenger's left toes. It took me a day to realize there's a separate connector for the Airbag ECU

and that's either above the passenger right toes or coiled up behind the trim panel to the right of the glovebox (one easy screw to remove)
The glovebox light makes a handy power source for the two croc-clips you have to power up on the 3-pin adapter. Just pop it down out of the glovebox "ceiling", black is negative. I had to extend the wires. And leave it open so that the light doesn't go off..
I can now connect to the main ECU and the software displays the ECU-model, reads error codes, but won't populate any of the data fields - I've no idea why, but am still working on that. I have the error "Lambda sensor integrator" - couldn't clear it. But the Check Engine light isn't on, so not bothering with it for now.
The connection was most stable when I adjusted the slider at the bottom of the screen down to 4ms - i think it defaults to 15 when you install the sw and with that setting it would lose contact after ~30 seconds. Thanks Alfan for showing me that.
Before connecting to the Airbag ECU connector, on my car, you have to select the "pre1995 airbag external sensors" button in the sw. It displayed the error code "short circuit at Vbatt" - I deleted the code and after maybe 40 seconds the airbag light went out and hasn't come on again yet.
Massive thanks to all contributors here and particularly to Wilco and Dimtey (and others?) for writing the code and reverse-engineering the serial protocol. Been there and it's not easy.
-Richard