there is at least one large EU freight airline that has for many years done exactly the same as Fedex is now planning
Are you sure though? I was under the impression that there was no 7th freedom involved and instead flights between the US and Central/South America are scheduled charter flights for Panalpina (and others were operated on behalf of Mexicana until its demise).
The closest comparison comes from talking boxes. Before 9/11, Iberia had a mini-hub in MIA connecting MAD/BCN with Central/South America. Planes and crews based in MIA, it was all connecting traffic, no local traffic allowed if I recall correctly.
(btw 5th freedom is the right to pick up in a country and transport to another country *via* the home country)