I've seen references to people doing it, or even with regular bricks, but haven't tried it myself. I could use a couple of fire bricks for another project, so if I ever get my butt down to Home Depot I'll pick up some extras and try it.
As an aside, one thing that's frustrating about reading about the clay saucer techniques is how many people try it without ever having done cooks with just an empty pan. There's not really a good baseline to show how much any of these options really improve things. I've done 225 degree cooks with an empty pan and it was just fine. Still, a little extra thermal mass would be cheap insurance.
I also wonder with a 22" if the extra thermal mass of the unit itself, the extra air inside, and the fact that I usually have a full charcoal ring to start means it's inherently more stable than an 18", at least if it's not windy. I haven't really heard any comments either way - this is pure speculation on my part since I've never even seen an 18".