Patrick, you're going to find some different opinions on this, but I believe that using the broiler to pre-heat the stone is a lost cause and that it only heats up the top of the stone, and, unlike the bake pre-heat, the heat doesn't really penetrate/saturate the stone- which is where you want it.
When members have taken temperatures of stones being pre-heated by the broiler, the top of the stone would get hotter, but the bottom of the stone would get proportionately cooler. You're only as good as the heat store throughout the stone, not just the temp of the surface.
The other downside to heating the stone with the broiler is that, if you've got the broiler on close to the bake, once you start baking the pizza and need the broiler, the oven will be too hot and the broiler won't kick in.
As Brian pointed out, you generally want to use the broiler during the bake, not for pre-heat.