Steve, from the Merriam-Webster New Book of Word Histories:

"Ye...was still commonly used by writers in the nineteenth century. Jane Austen, for example, in a letter of 8 February 1807 wrote '...& means to be here on ye 24th.'"

If my experience is any guide, "used by writers" means "used in handwriting rather than in print." I'm not so sure about that "commonly," but M-W is essentially the last word on these matters (aside from OED, of course, with which it is almost always in agreement).

In any case, if Jane Austen was writing "ye" in 1807, the argument that the ballad annotation must have been written around 1690 unfortunately goes out the window.

Perhaps there's another clue in the shape of the handwritten letters?