Have you considered using inorganic zinc to spray the underside of the car? I work in the paint shop of a company that manufactures natural gas compressors and are sold all over the world. Our off-shore packages require inorganic zinc as a primer. Or base-coat. Same difference. Anyway, that stuff is really good. Looking at the specs sheets and MSDS, the paint will hold up to direct saltwater splash.
Ameron 68hs zinc rich epoxy primer
American AZ76 inorganic zinc primer
International Interzinc 22.
Those 3 are zinc rich primers and are industrial grade paint for active off-shore oil rigs. Check out the pricing (if price is no object) and maybe you'll be able to drive your car directly into the saltwater, without worrying about rust!! Although the mechanics might go to shit, You'll have one shiny hunk of junk. LOL