There is some stuff made for marine craft called boshield I think. I could be wrong, it's been a while... Anyways, comes in a spray paint can and dries clear and kinda sticky for a little while. It's basically a lubricated clear coat that prevents rust on boats and things like that. You should be able to find it at a boat store. I have used it on shackles that I hang large speakers from for concerts ect.... When We do maradi gra at universal studios that rig sits outside in all the weather for a month or more, I coat everything in this stuff and it does a great job preventing rust. My only worry is I don't know how it would react to the heat of the exhaust. I will admit i have not tried this as i have not had a rust issue but I think it could work.
I've had my Roush axle backs for a good year now and I took them off at the end of November and looking to put everything back on for Spring, hopefully by the end of March. (I hate the Midwest.) :-( I did notice a small amount of rust on the welds, but no where else. I take pretty good care of them while they're on the car with using stainless steel cleaner and making sure if it rains or gets wet they're dried off in a timely fashion.