I think it would be enough if Syncthing provided a Rest API to access files, and other people can write platform-specific wrappers for that API (or extend existing wrappers).
There’s already GET /rest/db/browse, so we just need something like GET /rest/db/file for readonly on-demand functionality.
Edit: This approach would actually be preferable for Android, because we could include Syncthing directly in the system file browser.