I was considering something like this to have a local (or local networked) cache store for S3, I am doing 99,9% of readings, so it could be nice to put the cache in the S3-layer.
I think you can setup nginx proxy with cache. Unfortunately, that's easy only for public files and authenticated requests won't save you much (for that you can also write a small script to authenticate yourself if there's a cache hit).
Oh, yes, thanks for the suggestion. I will see if I can use NGINX as a proxy, all my files are public anyway.