Project files might be what you are looking for.
But direct access with JavaScript is not yet possible and is restricted (imagine a fraudulent HTML5 add having access to your hard drive, copying/pasting/deleting files there without you even being aware of it).
You might also want to check latest additions like Sprite: load from URL, AJAX request allowing arrays/dictionaries to load/save datas as JSON, etc... (you might want to check the changelogs of the latest releases of C2)