Polyglot, I have considered doing that type of modification but I've not investigated a supplier yet. Any sage words of advice you could offer? I'm guessing it would best have it's own thermostat and sensor. It would be nice (and safer) if I could tie it in with the float, but that isn't a deal breaker for me.
If you're about to buy an ATL, I wouldn't bother with an offboard PID. It's going to be pretty hacky.