What device is this you’re referring to?
A few months ago @forcerouge mentioned in another thread that he spoke to the marlin developers and they were open to the idea of programming this for him but didn’t have electronics side sorted out.
There has been a lot of work done recently on THC over at linuxcnc, all open source. If you’re up for it you can take a look at this code and see how they did it. I don’t know if it’ll translate well at all to the way marlin does things, and I know you said you’re really only looking for simple hysteresis type contol… but it might be worth taking a look.