Today I finally broke down and did it. I cooked a frozen pizza on the grill.
I put it on a piece of aluminum foil so that the ingredients would not fall through the rack. Since it is pretty close to the flame, I went with 100º less than the oven instructions. I would say that one could safely go maybe 150º below since I had a slight amount of burnt crust. Nothing inedible though. I also went with the low end on the time.
It turned out pretty good. I will experiment further to find the sweet spot, but I am pretty impressed with my guesswork this time around.