Re: "Aging" polished lacquer on an old horn
If you have a "bright shiny patch", I'd say that the full layer of lacquer was removed and now you are seeing bare brass. Over time it will darken and match better with the rest of the area. It will then become darker than the surrounding area that still has lacquer on it. All depends on the lacquer color.
Unfortunately, the only thing I can think of is to try to find some spray paint that will match. Maybe if it's lightly applied it will do the trick. Perhaps if you time it right, you can apply some clear lacquer on top of it about the time it matches the surrounding area. Could take months or a year or more for that to happen.
I have no idea if it would work to "seal the patina", but anything's possible.
Personally, I'd just leave it alone.