A further update, in case you decide to try this:
It seems this isn’t quite right; the images in the MP3 files are still occasionally checked (e.g., when you Get Info on the MP3 file), and if they go away the album artwork is deleted too. So steps 1 and 4 in my previous post are unnecessary (you can still do step 1 if you want a backup; just don’t overwrite the version that had the new image added).