In the current version of rclone its possible to access various S3 backends, but its not possible to serve a remote over the S3 protocol.
There is an oldish PR to achieve that: https://github.com/rclone/rclone/pull/6461
Our team has picked up on this great work, rebased the PR on current master and fixed some issues (S3 authentication & file name encodings).
You can find the current state of development in: serve: add s3 server - the second by individual-it · Pull Request #7062 · rclone/rclone · GitHub
We would be glad if people would test the implementation and give feedback.
The background why we are interested in this feature is, that we are in the process of building an S3 to WebDAV proxy, I will probably do an other post about it.