Inspired by @xafer’s comment in Dependency notation including the index URL:
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.