There is another (perhaps very far fetched) solution, which I'm throwing out there, nevertheless.
I've had several of my firearms "blued", locally. This is a cumbersome process, which involves stripping the rust, paint, grease, and other impurities from the metal, thoroughly. Next, the part to be "blued" is dipped in a tank, in which various chemicals, stabilizers, and accelerants are heat-boiled (I can get the details for you, and the exact mix / proportion which are used). This is then allowed to "cook" for several hours...is removed...and then cleaned, thoroughly, before a light film of anti-rust / protectorant is added. The entire process adds a light, thin, CONTROLLED layer of rust itself, which prevents further rust from occuring.
It also gives the "cooked" item a lovely blue-black gun-metal finish, which withstands extreme temperatures (think: the heat of a barrel, engendered by firing an automatic weapon at cyclic speed), is durable, and can be easily renewed / repeated. As far as I remember, the only problem which my armourer used to face was having a boiler tank of the right size. If an exhaust is "cooked" in pieces / segments, theoretically, I see no reason why it shouldn't work!
ANOTHER two cents worth of nonsense!
P.S. Of course...this will NOT prevent heat dissipation...but it WILL be heat resistant.
