THC does sound quite simple in theory, but I'm guessing it's more complicated than what it first seems.

Basically you need to know when the plasma is active, so you can then measure the torch voltage (doesn't campbell's use a suitable placed inductor, rather than direct monitoring?), smooth that voltage/signal to remove spikes/dips, then compare that to a reference voltage. Then depending on voltage, either signal the torch to be moved up (increase voltage) or down (decrease voltage).

Sounds quite simple, and I'm sure you could design a crude set-up to move the torch up/down, but getting something on the level of one of the commercial/pro-hobby boards would take quite a bit of work.
Would certainly be an interesting project for someone with the electronics know-how...