As per http://thread.gmane.org/gmane.comp.audio.musicbrainz.user/20271, am reporting this here.

Is it possible to add the new Archive.org wayback machine as an allowable cover art source? I note that it's in beta and I don't know if that's a problem, but otherwise it could be very helpful. For example, it will archive "on demand" (I think) which means cover art on obscure blogs is usable.