Yep, the board has a pad to access pin 8 with a pullup to 5v. So adding a momentary switch is already supported in the hardware, whatever the code does with it.
I'm thinking the most useful thing would be to cycle through multiplier settings, but the implementation would be tricky. Since this would be a performance control, it's important that it would maintain phase when stepping through multiply settings. So say I'm playing in a straight 4/4 time, using the 4 multiply. Then I step to 3 multiply to get a 3-on-4 feel. Then I click through to 2 multiply. At this point, at least if we assume that I stomped on the multiple select switch right on beats, the 2 multiply should be falling on the same beats as the 4 multiply was, except half as often. It shouldn't go out of sync when changing multiply settings.
So maybe the switch should simultaneously reset the wave phase and change multiplier? This would seemingly keep everything nicely lined up.
There's another concern that I think would be harder to deal with. If using a sine wave for example, it would be ideal if there wasn't a discontinuity heard when selecting multipliers and resetting phase. Ideally, it should smoothly blend from wherever the wave happens to be at the moment of the press to the beginning of the next wave. This might be as easy as a lowpass filter that's placed on the output of the wave only during the moment of phase reset. My digital knowledge is limited to DSP on the FV-1 (and just limited in general) so I don't know whether this would work for the hardware involved here.
Also, Chris, did either you or Tom ever address the thing I brought up a few pages back about where the start of each waveform table should be? Like, the sine and similar waves need to reset at their peak, but the square wave should reset at the positive-going edge?