Jens I really can't see how this is possible without implementing dedicated support directly for the "S3 REST API", which is something entirely different from the PHP local file system. In other words, to get something like Files Gallery to work with S3, it would need to be coded specifically for usage with S3.
For example, PHP can read the local file system in any way it wants, and get thumbnails and any other info (like image exif iptc). When doing this from S3 API, you are restricted to the API responses available for S3. For thumbnails, I guess you would need to load the remote image into memory before creating local thumbnails. Point is, it's an entirely different fundament.
This is definitely an interesting feature, but it's a big task. I really can't imagine any shortcut to get this working.