I think that you and/or your doctor should talk to an expert breastfeeding pharmacist, like this service:
Drug information centre - Southern Health
As they will have the most comprehensive information.

Motilium is used in the treatment of Parkinson's disease (to combat the side effects of some other medicines), and patients can take it for many many years. There may be a longer term study somewhere in the Parkinson's literature instead of the BF literature.

It does suck - you have done so darn well to get this far though, and you do need to make the best long term decision for you and your baby. Get as much info as you can though.