I recirculate my mash water through a coil in my HLT, I also have a 'bypass' ball-valve that allows me to bypass my coil. I find maintaining my mash temps quite simple -the HLT is kept at about 160, and I crack open the bypass AND coil-flow valves so that some of the flow goes through the coil in the HLT, some bypasses it and goes directly back into the 'tun -once I find the 'sweet spot', I leave it alone, kick back and keep an eyeball on the temps. Using the coil in the HLT as a heat-exchanger makes it a bit simpler than what you have in mind -for one, there is a lot of thermal mass in the HLT (I keep about 7 gallons of water 'at temp' in it, so the burner is turned down to the absolute lowest setting, or even cut off for a while). This has the effect of 'cushioning' the temperature variations. Whereas applying flame directly to your recirculation is going to be kind of tricky since the temp would change almost immediately.
One piece of advice, though: INSULATE! MY recirculation plumbing is wrapped with non-meltable insulation -you would be amazed at how much heat is lost in the copper pipes exposed to air. If you use a metal mash tun, insulate that as well (of course, if you are applying heat to the tun directly, that changes things). My 'tun is insulated on the sides and bottom, the lid fits well but isn't insulated.