Try and ask Hohner. As for plating services, I'm sure a place that does brass instrument plating can handle it no problem. It's quite common for brass players to get their mouthpieces re-plated to a different metal or because the plating wore off exposing some brass, which can cause allergic reactions in some people. Your local music shop should be able to handle it or connect you to a service. I think one of the main ones in the states that does this service is Andersons.
I don't know what the black coating is but maybe you can email Hohner in Germany or the US. Otherwise, maybe George could order you a Super 64 performance mouthpiece as it appears to be just polished stainless steel. I haven't compared both models side by side before with this in mind, and I only have the Super 64x performance which the mouthpiece doesn't give me issues of sticking. Since you asked, you should be able to polish the solid stainless steel mouthpiece to no limit. You only have to worry about over polishing if the metal is plated, lacquered, colored, and of course if there's a certain mechanical tolerance.
Boaz