BASF makes the finishes on the Hamburg Steinways. Obviously a vast company and I doubt they sell a "refinishing stick" or the like. There must be a good piano restorer near you-- worth doing a little digging. You in Dallas? Judging by your "handle." Good question. I know people have used high gloss black nail polish or similar but you can see the touch up in strong light.
Piano black polish and refinish
Is there a kit or a very good tutorial for the "normal man" to fix/repair/refinish piano black speakers? I figured surely there's a good kit with the right compounds and etc made for pianos but the only real advice I've seen is a complete rebuild or taking it to a car paint shop.