O.K..lets get creative.
I have baked enamaels sucssesfully onto metals. My chainring is custom flat black.
The procedure is enamael @ 120-140 degrees 20 min.

You could take the cluster apart, steel wool the rust off the cogs, paint, hang in the oven, bake, re-assemble.

The only part of the cog that would continue rusting would be the part of the teeth making contact. Use wax lube.

Get a new chain maybe.