I use the coolers too, I use an electric HLT and an external heat exchanger to heat the mash.
What sort of temp controller do you have on the element currently?
The issue is that the mash has some serious insulative properties, making it necessary for a mash mixing device, or at the VERY least (you would still get hot and cold spots) recirculate the wort that is in the mash to keep it moving.
Even with a PID, I think youd have issues with the mash temps not being uniform, because of the sheer power output of the element, insulated in the mash. Even heating a tank of pure water takes some agitation to avoid temp. stratification.