Let's see if any other users are interested in this. I'm a little reluctant to add new sync methods as they have a heavy overhead in terms of maintaining and supporting them. beorg can sync with a folder from another app - so if you have any app which does the S3 syncing you may be able to use that with beorg.
Joshua Damiel Chan
Any chance of supporting S3 as well, in addition to Dropbox, WebDAV, etc.? Happy to pay for it as an extension of course.