I found this old info. it seems to show that there has been argument from the beginning on what people were doing to improve the breeds (or to win a prize). There is reference to a Maltese being used and breed purists showing disdain for it. I don't think it does anything to settle any questions, but does show just how long this has been going back and forth. With so many dogs being shown in the broken-haired Scotch terrier class, it no wonder that there are so many variations within the modern Yorkie.
The country - Google Books
Joan Gordon has a great history of the Yorkshire Terrier posted on the YTCA site. She makes a convincing case for there being no reason for a Maltese (or other breeds that have been alleged) to have been used to produce the Yorkshire, but that doesn't mean it wasn't done by some breeders. Here's a link to that history.
History of the Yorkshire Terrier by Joan Gordon