Clay,
Yes, you could use the cruise switch, but I believe it is the opposite logic of the bottom switch. It grounds the one wire when the clutch is pushed in, instead of it being grounded when the clutch is out, as the starter interface switch does. However, I CAN reverse the logic in the chip, if you are game to experiment. You just need to figure out which wire is which. According to the schematic, there is a black wire, and a green wire with a black stripe. The black wire is ground, so the wire you want to cut is the green/black striped wire. You install the diode the same way, with the banded end toward the switch.
Let me know if anyone wants to play with this.