As far as I can see we are talking the Chitlins Circuit here. We all know, various cities in the U.S. had clearly identifiable musical styles be it Blues,Soul,Jazz or whatever. Therefore I have concluded that it's pretty obvious that the term "northern soul brothers" could have been used to describe artists from New York, Chicago etc. AND their own particular sound. However I can't find any evidence of the term being used, other than in the British sense.
By the way the Chitlins Circuit is nothing new to many British Soul lovers its been known about for years. Anybody who took any time at all to research the artists, who made "northern" (British usage) soul style records, quickly became aware of the Chitlins Circuit and its role in Black American music history.