Using a virtual file system for library source code

We are building a feature we're calling Working Set. It involves externally building jars for the parts of the codebase that you're not planning to edit (i.e. those outside your Working Set), then adding them to your IDEA project as library jars. This speeds up IDEA immensely, because your project sources contains very few files, and the rest of the code you depend on (many thousands of classes) are just libraries.

We're not sure how to handle source code for these jars. We can't attach it directly because of IDEA-16227. In the past we've built zip files containing relevant source for library jars. However I think it might be more efficient to use a virtual filesystem. I imagine its URI's would look like backingsource:///usr/local/blah/myproject!/java/com/example/whatever/folder!/com/example/whatever/folder/SomeFile.java.

My question is, is this kind of use of VFS supported? Do you guys have any other ideas for how to implement this? I know in the past I've filed bugs for DummyFileSystem which were ignored. And last I checked it was tricky to register a custom VFS.