Possibly try asking a pharmacist, especially the one who fills your Mestinon (others might hesitate to advise you since you aren't really their patient), but they might just say ask your anesthesiologist or other docs.

Seems like most drugs that would help the cramping are going to work against the mestinon. I'm thinking it would have to be something that stayed localized in your stomach. So I'm thinking maybe you can't stop the cramping, but you could do stuff to feel better like take pain meds / relaxants? If those meds don't cause you problems.