If GUEST has an Anglo-Saxon source for it, let's see it. I don't believe there is one.

There is an early source for the John Barleycorn story that puts a different spin on it. It's a letter from a Flemish painter of the 16th century, reprinted in Letters of the Great Artists (ed Richard Friedenthal, 2 volumes, Random House 1963), written to accompany a gift of some wine (I think to the city fathers, who he was trying to bribe to get a commission).

The two differences are (a) he tells the story about grapes rather than grain and (b) the genre he is parodying is Protestant martyrology.

I don't have access to the book at the moment but the letter is about two pages long and easy to locate.