How do I set up a Document based app that uses multiple files?

I want to have a custom project file that a user selects in a UIDocumentBrowserViewController. That project file would then reference files in the same directory which would be opened and written to programmatically. I can't seem to get access to the project's sibling files programmatically (presumably due to sandbox restrictions). What's the best way to get this to work? Alternatively, I add a custom extension to the containing folder name, so it appears as a single file with the custom project file and the other files inside of it.