This would be useful to access native APIs and build a offline cloud 9 that works on chrome OS and mobile devices.
Could you explain in a bit more detail what do you mean?
There is a desktop version of cloud9 using nw.js https://cloud9-sdk.readme.io/docs/running-cloud9-desktop, and there is a simple demo of vfs server working with localStorage (http://c9.github.io/core/).
In principle it is possible to modify ide.offline.html to use chrome filesystem api, but there is no way to run the code on chrome OS, which makes usefulness of such an app rather limited.
But I agree… for the vast majority of the most common use cases it would be pretty pointless.
Perhaps at some point in the coming years a reasonable VM setup on ChromeOS+Android releases will allow for something like that; offline single user, online pair or team programming. But that’s wishes and hope at this point.