You can direct heat the MLT if the mash is thin and you stir. Otherwise, the heat will create magma pockets and uneven heating, and killed enzymes.
I have one, and when I do single infusion, I use a program to calculate the strike water temp prior to dumping the grain in. You probably have one of those. The stainless steel holds temperature pretty good, as long as the grain is in there. But if you remove the grain, like for decoctions, the temperature goes down, especially in cold weather. But the good news is, with most of the grain removed, you have a very thin mash, and you can direct heat to raise the temp back to where it was, or even to a higher level if you want.