Google has introduced browser APIs to allow browser-based apps to request access to the user’s local files stored on disk.
The surface area of this API is fairly small. (See https://wicg.github.io/file-system-access/#idl-index.)
remoteStorage.js (or some other library) could implement these APIs in such a way that, provided the user has (a) indicated they wish to store their data with their remoteStorage provider rather than granting read and/or write access to the files stored locally on their disk and (b) has connected and authenticated with their remoteStorage provider, the data that the operates on is a remoteStorage without requiring the application or its creator work with remoteStorage.js APIs or the client-to-server interface exposed by remoteStorage providers.
It would also be possible for the library that implements this to be packaged into a browser extension that intercepts all File System Access requests and offer the user the option to confine and redirect the application’s reads/writes to a given folder with their remoteStorage provider.