I think some of the sprays that the well informed posters above talked about are probably pretty durable, especiall after several coats. The bakelite material is porous enough (why it dulls in the first place I think) that a clear spray will adhere very well for a good while.
If the spray option is not for you, I think the material can be polished. I know the Polypropylene agitators of the later 60s and forward can be for sure, however they don't dull as much.
You would need to find the right compound and a good buffing wheel, etc. but I feel strongly that a small surface layer of plastic AND the build-up of minerals and detergent residue could be buffed off, sort of like buffing out an old car's weathered paint.