How about assigning an additional genre like “Christmas Songs” or “XMas” to those songs, and adding a ( … ) AND (NOT GENRE = ‘XMas’) (from memory, syntax doesn’t have to be 100% correct) to the smart playlist?
Hmmm… does the Firefly database support multiple genres for files? I only recently started updating my tags thus and haven’t looked it up yet (if not, you can always use the christmas genre as the main genre for the files).