MMN isn't its own specialty since it's rare but any MDA or ALSA or other good ALS clinic/neuromuscular center should have physicians who are familiar and can do that differential. Boston has several; there is also Yale, New Britain, U Mass in Worcester. And yours in Providence.
Yes MGH certainly sees MMN in their neuromuscular/ ALS clinic. I think that is where MMN is usually seen as it is so rare you are not likely to find someone in practice that only sees MMN. They briefly thought I had MMN and my MGH ALS doctor was ready to treat me and the NP spoke generally about their MMN patients as a group they see