Encryption should happen client-side [1], an S3-to-S3 gateway wouldn't help unless you deploy one on all client nodes.
s3cmd and duplicity has support via GPG, but not all S3 clients will know what to do with those files.
[1] http://www.skylable.com/blog/2014/09/transparency-reports-se...
Disclaimer: I'm co-founder of Skylable