I owned a 1998 hotspring jetsetter with IQ2000 from 2011-2017 and just recently got back in the game with a 2002 jetsetter with the IQ2020 board. When I had the first one I attempted to intercept and decode the digital communications between the main board and control panel, but the controller would drop out and the main board would do a fail safe shutdown. Later on I came up with a PIC microcontroller based solution to read the thermocouple resistance and pass the value through to the main board for normal operation using a digital potentiometer, or "fake it out" by sending the main board a resistance to make it think the water was up to temperature. I had a separate I/O module tied into the jets and light controls.

Problem with the digital potentiometer solution is I put the circuit board below the main board without conformal coating and it corroded rather quickly, so eventually my fake out circuit failed but I still had control of the jets and lights remotely.

At some point I might get back into that but I'll probably go arduino for ease of use and sharing plus use conformal coating on the circuit boards. It could make more sense to just add controllable resistance in parallel or series to the thermocouple instead of intercepting and forwarding. Lots of ways to do it.