Ah, yeah, it would need to work with an HTTP server for this to work well. Sandstorm has persistent storage on the server side - think server-side code running in a Docker container with client-side JS restricted to communicating with that container on the server. Client-side JS can't interact with the filesystem.

I guess I was assuming that real-time collaboration would eventually involve some sort of synchronization server as a rendezvous point, in which case that would be the natural thing to run on the server side in Sandstorm. That may not be the route you're taking though!