Baking soda worked surprisingly well!
For the horn itself, I simply let it sit in a tub of vinegar. This also worked extremely well.
Here are the before and after photos of the bell:
To further polish the body and get into all of the hard to reach spots, I used a metal polish and an old T-shirt. You can even see the camera reflected in the post below:
Not bad! It's always really satisfying to see an old horn transformed by a good cleaning and polishing. The contrast between the burnished and satin finishes, common on horns of this vintage, is especially striking. Not only does it look great, it doesn't smell musty any more. Now, all I have to do is keep my fingerprints off of it while I put it back together!