With the removal of fs_simple_objects so all content is now in the content-repository, file storage is close to being capable of browsing any CR folder and in the back of my head I've thought about doing that at some point.

Sort of turning your question on its head.

It is possible for other applications to store docs using File Storage but it's not much more difficult to just store stuff in content folders directly.  The .LRN homework package uses file storage in this way, poaching the UI in modified form too.  If you don't need the file storage UI I'd suggest just storing directly into the content repository, file storage just derives a simple subtype and provides some more API than the CR does.

I assume a DAV interface would just hook up to the CR, there's nothing in the subtype created for file storage that a DAV interace would need to know about.