The question I would ask, is why not just pre-heat the oven the normal way?.

Broilers can be left on for 30 minutes, but they will naturally cycle on and off- at least if the door is closed.

I suppose if the oven is pre-heated and ready to go but your pizza steel is cold and you don't want to wait another 10 minutes you could use the broiler (that would take maybe 6 minutes- and then turn the oven back on-ugg ) or you could heat up the pizza steel on the stovetop and then put it in the pre-heated oven.