How about this.
I buy this One Relay Controller off ebay:
USB One Relay Controller RS232 Serial Controlled | eBay
I follow these instructions to install it:
KMtronic USB relays from Mach3 - step by step
I remove the green and yellow (I think that's the right color - I'm working from memory) wires from the CandCNC THC Sensor PWM Module that are supposed to fire the torch and hook them up to my new USB relay.
Does anyone see a problem with this? Am I missing something? Some kind of conflict I'm missing? A reason why it won't work?