Hi Sean,
Mach3 is obsolete and unsupported. Consider using KMotionCNC instead.
None of the DoPC functions will work because those are only supported by PC Apps that support them. Mach3 does not.
There is an option for a Cycle Start Bit in Mach3 Dynomotion Plugin Configuration.
I would think the XPLUSBUTTION would work and Jog at 100 steps/second but then it would never stop. You would need a Jog(0,0) to stop when the result is 0.
In Mach3 to do the other functions you would configure the Ports and Pins | Input Signals | OEM Trig to your KFLOP Input Bits, Then assign an appropriate OEM Code to the Trigger under System HotKeys Setup.
Please post any C Code using the #Code tags to make it more readable.
HTH