Well that there is a whole different animal. I have heard people talk about, but never actually confirm that an aluminum lock nut (instead of a stainless steel one) also works. Aluminum isn't as reactive a metal as magnesium, but it is used for water heater anodes as well.
Having never built a RIMS tube before I don't even know if you attach the element on the inside with a locknut or whether you thread it right into the pipe so my second suggestion could also be worthless to you!
Food grade silicon could be something you could try in your situation. I've heard mixed reviews on it. Some people say it works great, but others have complained it doesn't stay on very well on the base of the element.