But... thumbs.db is precisely not an "attempt to use filesystems in "interesting" ways" — it's literally a just hidden file with previews stored in it. Storing the preview in the alternative stream of the file with the picture itself would be "an interesting way".
To me it seems more sensible to store information relevant only to this OS in a specific cache somewhere within that OS. It would even make cache-like functionality such as evicting old entries super easy.
