To make it easier for application authors to migrate to remoteStorage (and make it easier to author an extension that intercedes on app-to-cloud-bucket calls to read, write, and/or copy to a remoteStorage account of the user’s choice), the/a remoteStorage library should offer an S3 compatibility layer.
(That is: all on the client—not an extension of the HTTP APIs that a conformant remoteStorage server should implement. It should also be possible to ask remoteStorage.js (or whatever library implementing this) to provide a custom fetch and/or XMLHttpRequest implementation to overwrite the browser’s native fetch/xhr.)