I was just having the same thought about this—the redundancy of downloading scans from Internet Archive and re-uploading them to MusicBrainz via Internet Archive—which it seems like it would be best to avoid.
It would be great if there was a way to just provide the main Internet Archive URL on the MusicBrainz release cover art page and parse it to link to the existing files—with a virtual coverartarchive.org URL, similar to what is already created—instead of duplicating them.
Not sure how to have more of a discussion about this, but it seems worth having because there is just so much artwork available. I found the other thread first, but it doesn’t really address the redundancy issue either.