While I think that some extra thermal mass is a good idea, cracked fireclay on a sloped surface is going to slide off. Welding a lip, as Craig mentioned, would support the fireclay, but... welding stainless is a major pita.
I've actually been contemplating a way of keeping refractory on top of a sloped metal roof for some time now. Something that just came to me is taking heavy duty steel wire and forming it into a big loop. When your form the fireclay on top of the roof, make it into sections (like slices of a pizza), forming a flat exterior edge with a groove for the wire. With all the pieces in place, the wire acts like a harness, preventing the refractory slices from sliding outwards. The slices also need not be too form fitting to make room for expansion.
You probably wouldn't want to drive with the slices in place, but it should be easy enough to assemble/disassemble. I'm thinking maybe 1" thick refractory wedges or possible some perlcrete that's a little heavier on the 'crete to give it more thermal mass.