This is my first time hearing of Sandstorm!
Their page https://sandstorm.io/how-it-works writes:
> For example, when using Etherpad – a document editor app – on Sandstorm, every document lives in a separate container, isolated from the others. The front-end and database for that document live in the container. The container has a private filesystem for storage. JavaScript running in the user’s browser can talk only to the document container for which it was loaded, no others. All of this makes up a single “grain”.
Right now Athens works directly with the filesystem (and therefore filesync services), not a REST-ful/HTTP server. I'm not sure how Sandstorm apps "talk to the document container." If they allow read/write access to the filesystem from the web app, it could work, but I'm not sure how Sandstorm works.