Here's a list of backup quarterbacks that BYU has faced in the Mendenhall era. I added some additional ones in my response towards the bottom of the thread.
http://www.cougarboard.com/board/message.html?id=12871965
As for who else torched BYU, Jeff Ballard (TCU) and Steve Levy (Cal) in 2005 come to mind. As I noted in my reply to that thread, Air Force's backup also had a field day against BYU that same season.