"There was all kinds of trade and sea-travel all along the east coast of North America (incidentally, a busy trade between Nova Scotia and the Caribbean); all it would take is one singer from the Maritimes meeting one singer from the Appalachians for a song to be passed along (I'll spare you the obligatory colourful description of the imagined meeting ...)."

Fair point, 'meself' (I suspected someone here would be more knowledgable than me), but like Lighter I feel the odds are long.

(a) Sharp collected the song over 400 miles from the coast, in pretty remote mountain backcountry.

(b) He collected it only once (compared with 30+ examples for many other British ballads that are older and would have been brought over in the early mass migration), so it seems to have been relatively rare.

(c) I should more precisely have said "well-established migratory routes across the Atlantic" to both Virginia and Nova Scotia, which would represent a more plausible avenue for song transmission than a chance meeting between singers at sea or in port.