Sarah, you're spot on with the John Fahey albums. The medley of Hark the Angels Sing and O Come All Ye Faithful has the distinction of being the only Christmas song I play off-season.
I was thinking about this the other day since I work in retail I'm subjected to this stuff for a solid month (or more). I was thinking of bands and artists past and present that could've made amazing (or at least interesting) Christmas albums— albums I probably would buy— but had the good sense not to.
Example:
If Kraftwerk made a Christmas album.
If Devo made a Christmas Album (I surprised they didn't actually, it would seem like a logical ending point for the band).
If Ted Leo made a Chirstmas album.
One last thought, the worst thing you do to someone in the middle of July is wait for them to get into an elevator or car with you then start softly humming "It's Beginning to Look a Lot Like Christmas." Guaranteed to get stuck in their head.