Currently it is only possible to mount S3, Google Drive, Dropbox etc. as external storage in a subfolder.
Mounting S3 as primary storage is implanted in the Enterprise Edition. This could be interesting for the community too, so I think we should rewrite it. What do you think?

Reference: https://doc.owncloud.org/server/8.0/admin_manual/enterprise_external_storage/s3_swift_as_primary_object_store_configuration.html