I put together a tool that does this with local directories.
This is of course at alpha quality at best, although there isn’t really much that can go wrong—there’s no caching, no mirroring, no nothing; requests to non-local sources are transparently redirected with HTTP 302. I did some quick testing and things seem to work as expected.
I know very little about S3 and other web storage stuff, so it’s probably best to leave this in others’ hands.
I took a quick look, and would suggest you probably want an explicit extensibility/contrib mechanism, so that other can easily build the wrappers they need without having to force it all back onto yourself.
But as you say, there isn’t much to it PEP 503 is a pretty nicely designed system.