Thank you. Why not? We would love to see it, but we're also not actively using Amazon S3 at the moment. But we would be more than happy to review code contributions.
Adding a new filesystem to Drake's source is very easy. You just create a filesystem object that implements a bunch of methods for: listing directory, removing file, renaming file and getting file's timestamps, and then put it along with the corresponding prefix in the filesystem map. That's pretty much it. Assuming there's client JAR for Amazon S3, written either in Clojure or in Java, it should be quite simple to do.
