This should be implemented, though it is not trivial.
The user is allowed to force scrobbling a song marked as disallowed. This does not trigger a reprocessing, nor should it.
This means that we cannot actually rely on the pipeline for this, and have to move the logic out to lazily load it as needed.

Most likely this means implementing an abstract function on the BaseSong class named something like fetchTrackArt.
This function will be checking if there is anything in parsed.trackArt or metadata.trackArtUrl.
If those return null, then fetch. If nothing is returned we explicitly set metadata.trackArtUrl to the default art to avoid refetching.
In the clonedSong implementation we must also then send that trackArt down to the main song in the content script, as the fetching may (and usually will) occur in a clone in background or popup.
song.getTrackArt can then call fetchTrackArt as part of its implementation.
on top of getTrackArt itself having to call it, it must also be called in the webhook scrobbler before the song is sent.