SD Shad, it is my understanding that the Lomaxes were on the official payroll at the Library of Congress for only a small part of their collecting history. No idea what their contract arrangements were; never seen any details.
John A. Lomax was connected with Harvard University for much of his collecting career; he was only an "honorary consultant" to the Library of Congress. The Library was a repository; they probably retained all copyrights except for material specifically issued by the Library (and the Smithsonian). Alan Lomax renewed copyrights on John Lomax material.

Probably a complex question you are asking here.