Hi again folks - bumping this up again as all of my searches have turned up nothing.
I've gone over the boards that I have and tried to find what the MAME source code refers to as the "Inputs" dipswitch. From what I can see there is nothing on the boards with this marking.
Experimenting within the emulator suggests that when the cab type is set of "Deluxe" via the menu, there is no modification to the Program Roms.
I've purchased some more manuals from Japan to see if there is any reference in them to this but alas no. (Although I did find a manual which talks about retrofitting the Namco force feedback steering!)
I've tried to email the folks behind the System 22 driver to see if they can explain how the feature functions but haven't had any luck.