I just came home from work to find an e-mail from a the global community lead at Bethesda Softworks. He said "While we don't give formal permission for that sort of thing, I don't see it being an issue."
So, yay! That is our processional and recessional music confirmed. :) Now we just need to hear back from Blizzard Entertainment. Once my mom books the DJ I will just have to confirm with him that this is sufficient permission for him to play the pieces.