How to write SCSS function to access file system?

I'm using Broccoli (which uses libsass) to compile SCSS code for an Ember project, and I'd like to create a SCSS function that reads a named file from the file system, processes it, and returns the result.

So far I haven't been able to find any references on writing a SCSS function that calls out to the file system.

How do I call out of a SCSS function to access the file system when using Broccoli?