Jim, if you want an LDS example of a reference like you believe Matthew is making referring to Isaiah's lost words, here is one:
In the BOM, writings of some prophets are referred to, but aren't currently known to exist.
"These writings are of Zenock, Zenos, and Neum (1 Ne. 19:10; Alma 33:3–17). An extensive prophecy by Joseph in Egypt (which is not in the Bible) is also apparent from 2 Ne. 3:4–22, and a prophecy of Jacob (not found in the Bible) is given in Alma 46:24–26. These writings were evidently contained on the plates of br*** spoken of in the Book of Mormon (1 Ne. 5:10–13)."
http://www.lds.org/scriptures/bd/lost-books