Mike, steel's conductivity makes it unsuitable for grill use. Grills, because the heat source is on the bottom, tend to bake the bottom of the pizza much faster than the top. Because steel speeds up baking, it exacerbates this issue.
Except for perhaps a grill with a rotisserie IR burner, there's no grill configuration that could possibly make steel work as a hearth.
Cordierite would be nice, but if you can't track it down, then I'd look for flat terra cotta tiles. Anything that's flat and ceramic, as long as it doesn't contain lead (most commonly found in glaze) will work. Even if you can get a regular flat brick, that might work, although I'd look for brick splits (1/2 the thickness), and, even with splits, it's going to be a long pre-heat/a lot of fuel. Typically, brick splits take about 2 hours to come up to temp.