From what I can gather, S3Proxy[1] can do this, but relies on a Java library that's no longer maintained[2], so not really much better.

I too think it would be great with a simple project that can serve S3 from filesystem, for local deployments that doesn't need balls to the walls performance.

[1]: https://github.com/gaul/s3proxy

[2]: https://jclouds.apache.org/