So how do you go about making the pedal / accelerator a button? Manual INI editing? The MAME UI doesn't seem to want to accept the mapping when I press it.
Analog input ports have three inputs. The first, "xxx Analog", is for mapping analog devices to the analog port. The other two, "xxx Inc" and "xxx Dec", are for mapping digital inputs devices to the analog port. For example: an analog input, say analog joystick, would be mapped to "xxx Analog", say the x axis. OTOH, if you only had a digital 8-way joystick, you would need to map joyright to "xxx Inc" and joyleft to "xxx Dec". Mame will not accept digital inputs to the "xxx Analog" part, so...:
For pedal, just map your button to the "Pedal x Inc".
One thing to watch out for is mame automatically converts an analog device's input to digital if mapped to any digital port, including "xxx Inc" and "xxx Dec". If your analog device isn't acting as well as you expect, check and make sure the analog axes are NOT mapped to the "xxx Inc" or "xxx Dec" ports.