At least two books have used the phrase as a title:

Book Clip: Google Books might be related.

Another book with same title, different author (John-A-Dreams)

Both of the above merely use John of Dreams as titles, but a little look inside might give a clue at least how the title was chosen. The Hamlet reference you already know is probably a likely source, which wouldn't add anything to what you already have.

I didn't see anything really informative in the bits of the two that I read, but I didn't go far into either.

Beyond those two, Google Scholar search results degenerate into psychobabble books about dreaming etc.

Scholar search for "myth John of Dreams" gets mostly junk about some guy named John Legend. (a common Google breakdown - the popular blocks getting to the interesting)

John