hi, if you look at the API explorer (projects and documents collections tabs) there's not a way to do this.
Even a solution using an exists statement is a little restrictive, since it will not report on empty folders -- it relies on there being a document in the folder in order to make the filter work (see the line stating that the document's project ID is equal to the ID). If you were interested in reporting on folders that didn't have files already in them, the filter wouldn't work. As long as you're only interested in reporting on occupied folders, you're probably ok... probably. (I'd recommend you run some tests on sub folders to be sure)
Similarly a view probably isn't possible, since first you'd need to report on a collection of a collection (folders are a collection of a file) and this is the unpossible part. And second -- you would also need there to be an uploaded file.