RTV as muffler patch?

I can relate and am a cheaper bastard. Sometimes you spend days fixing something right then an idiot totals your whole vehicle. Repairs are a moving target.

There are epoxies good to 750 F, but usually just industrial supply and they are more brittle. Silicone is good to ~600F, but doesn't usually stick well, so better as a gap filler. I think the muffler patch material is a ceramic, similar to "furnace cement". I think you can buy it to secure wood stove woven gaskets. It is like pottery glaze, an aqueous solution of oxide powders, similar to fine mud that dries hard. Like glaze, it needs to "fire" to drive off the water (hydrates). Otherwise, it will dry hard but eventually pit as atmospheric moisture attacks it (little of that in the OC). But, it will also fall off like mud, so needs fabrics to hold it. Those are usually woven fiberglass (~1000 F) or alumina (~3000 F).