Neither "(A)way down in Dixie's Land", "Dixie, Jr." or "W.S. Hays" is not mentioned in Nathan's Dan Emmett and the Rise of Early Negro Minstrelsy (according to the index of the book).

There's an article online:
"Dixie" and Its Author by Robert Sheerin (The Century; a popular quarterly. / Volume 50, Issue 6, Oct 1895, pp. 958-960). The manuscript reproduced on page 959, which is also in Nathan's book (p. 249), was written much later ("It is generally believed that the original manuscript of the song is lost. All the manuscript copies now extant were made by Emmett in the nineties and later, when he lived as a pauper in Mount Vernon, Ohio." -- Nathan, p. 250).

~Masato