Or even if there was a way for them store an image, like album art, as a file in a filesystem. They could keep a folder with multiple files...

That might be confusing, though. Too bad they don't have a way to show a directory as a single "package" in the Finder. I can imagine that would be useful for a lot of things.

They ditched the concept about ten years ago. Being out of the loop I had to look up if APFS even supported them anymore, and in doing so, found opinions on two ends of the spectrum: the concept was the best thing ever, or one of the most detestable features. I guess no one really understood how to use them anymore and added to it's demise.

The closest there is for media files: http://m-tags.org/