Brass is easier to strip than plastic, you don't have to worry about the stripper eating the brass. In an ultrasonic cleaner you might try a good strong solution of washing machine detergent (Tide) in water. We had some amusing experiences cleaning plastic control panels years ago. We ran out of our usual cleanser ( "Detergent, Aircraft Washing") and to get on with the job, used Tide instead. The Tide took the paint clean off the control panels, turning a flat black panel into a clear piece of plexiglas.
If Tide isn't strong enough, you can go to lye, but you want to be real careful with it.
Plain old hardware store paint remover ought to do the job on brass, but I'd be reluctant to put it in an ultrasonic cleaner that I cared about.
David Starr www.newsnorthwoods.blogspot.com