Searching briarhopper resulted in information about a radio show from 1935.
https://www.historysouth.org/briarhoppers/
Eventually, I did find an entry in
https://www.daredictionary.com/view/dare/ID_00005809 1966 that references the slang term. "Brier hoppers. Poor hill folk (hillbillies) who come into Ohio from Kentucky and West Virginia to make money. Usually live on credit." It sounds like it is Ohio slang rather than broadly Appalachian.