Brekelefuw

We have a large scratch brush wheel for our buffing machines at school. It is a large wooden block with a threaded taper to mount on the buffing machine. In the wooden block are long brass bristles that are really soft.
To do the inside of the bell I used the scratch brush as best as I could, and I also bought two different shapes of brass brushes for my Dremel tool, although I found that it wasn't nearly as consistant of a finish, and took much longer to do.
I have seen similar looking brass brushes at the local hardware store for very cheap that could be mounted in a bench motor or drill, but I don't know how stiff the bristles are. You would want to go with something very fine so you don't damage the instrument and remove too much brass.

I have read that Scotchbrite pads work decently for this as well. I think the fine grade ones, but I have never tried it.