My argument, when the time comes, will be that I think only game screen, movie characters, and official character representations (ie- bungie commercials) should be acceptable. Landfall being treated as a mini movie, and therefore included.
I hate the idea of concept art being adequate, especially since you won't have enough views to do both sides of a costume, and because it's an artistic rendering that ignores concepts of the absolutes that appear in the "finished suits."
I mean, it could be considered non-canon, but again, without establishing shots of both sides of the costume, how could it ever be determined to be considered canon accurate? What if there are two folks going for that costume, and both look completely different from other views; where is your "canon" now?
The easiest "background" characters to substantiate for reasons of being considered "Canon" are going to be the HALO 3 ones you can get enough in-game screenshots of.. like the traxus uniforms and Section Personnel for example. Halo 1 and 2 are slightly harder to do, but still achievable. The Landfall minifilm is pretty good too.