A few years ago, I was sure that Simonites were selling polyurethane Bump stops for series land rovers. I think they were made by Deflex..
Unlike the usual rubber bump stops with their bonded steel backings, these just trap mud, rust, and destroys the chassis or any galvanized protection,
The poly' bump stops I saw had no steel backing, and the bolt holes were molded into the polyurethane buffers themselves.. And had no steel backing.
My question is, does anyone have polyurethane' Bump Stops and what make..
BTW just seen Polybush are now offering Bump Stops for series land rovers

Wet and crud gets in behind poly bump stops just the same promoting corrosion. Not seen any that do not have a metal plate in them, after all there has to be a fixing. Standard stops are tough and cheap just a good smear of grease on the plate when fixing keeps the rot away.
Even our dog who can destroy any so called " tough" dog toy in minuets can't trash one [ back plate removed ] in under 6 months.

My only point was, wet mud, water and crud against a galvanized steel surface has minimal effect. The zinc coating protects the steel. The same water and crud trapped between the steel backing of a standard bump stop will cause rust and eat the sacrificial Galvanized layer away..
I'm sure I did see polyurethane bump stops with the fixing holes moulded into the buffers..
But like you said, if Poly aren't available then a good barrier between the steel and chassis would do.

Here on the Welsh coast salt is the big enemy, even galvanized gates go brown in the sea facing side. Salt on roads in winter is a source of corrosion for many in other parts of the country.