Is the top piece that has the gauge in it moveable or is it fastened down? Possibly bleed some heat off with lid slightly ajar?
It may not be necessary. I just did a quick test. I ran the oven for 20 minutes with with the inline regulator opened minimally and the dial on the oven turned to "Max". The temp readings are as follows.
-directly on the center of the bottom stone is ~400F
-center of the ceiling stone reads ~450F
-General temp reading of the heat coming out of the mouth of the oven is 435F.
So yes, this oven should be able to do lower temp bakes as well. Chicago thin, DD, Detroit, Sicillian, American, etc.
Edit: I have both the 8"x10" and 10"x14" square steel pans from Detroit Style Pizza Co. Only the smaller pan fits and rotates freely on the stone.