I wanted to add Content-Encoding:gzip to my s3sync.rb headers, but I didn't see a way to do it, so I modified the code to accept a --headers parameter which accepts arbitrary headers.

Example:
s3sync.rb -r ./testdir/ mybucket: --headers="Content-Encoding:gzip Cache-Control:public Expires:'31 December 2037 23:59:59 GMT' x-amz-acl:public-read"

Please be skeptical of my code, see attached file.