Very jankily! I just started using Gen (or C for that matter) this week, so I'm sure there's an easier way.
I have an external tap that goes tip to sleeve and it outputs a small audible click into the signal once when I press and a quieter one about a third of the volume when I release. So I multiplied the signal enough so that the first click exceeds a threshold while the other doesn't and it outputs a 1 for the duration of the click (about 4-10 samples). I tested it for awhile and it worked very reliably with my footswitch, but I changed the B Knob to attenuate the sensitivity for other switches, haven't tested, though. I then made sure that it would ignore any and all "1's" for 50 ms after the 1st detected sample (no way can I manually press it that fast, so it should be sufficient) creating a bang for all intents and purposes. And from there I just used what was laid out in the Tempo Delay tutorial Rebel Technology put out. It worked a treat!