I think I've seen ONE legendary artifact in an SOS (and only 300 or so luck), and it was in the first Zipactriotl Quest MIB I opened (which came out as an Ancient SOS) in Trammel. I have it locked down in my house. I didn't realize just how much of a fluke it was, and was disappointed when I never got an ASOS again from the Zippy quests.
Of course, the legendary (a ring) was totally unusable due to misplaced and unaligned properties.