I would also like to see this implemented! When viewing my audiobook library, I just want to see the "author name" folders and, under them, the "book title" folders. I don't want to also see the mp3 files within the book title folders (a single book may be made up of 50 or more mp3 files). If I can just browse a tree showing only author folders and book folders, that makes it much easier to pick my next audiobook. Thanks for listening. Directory Opus rules!
P.S. Pls forgive if there's already a way to do this. If so, I hope you'll explain. 
UPDATE: Wow! I just found this custom toolbar that displays folders only in flat view (and has other useful flat view buttons). This is basically what I need. Thank you to the collaborators who created this. And hats off to Dopus for being so powerful and versatile.