Thanks, Chris! I totally forgot to mention that file systems like btrfs give you many of these features in a clean, integrated, fast manner. btrfs makes more sense for a significant subset of the use cases proposed for unioning file systems, especially cases where you'd like to tentatively make changes that you may want to make permanent (or roll back if they don't work).
One use case I don't know how to solve with btrfs alone is the writable-over-DVD case. I suppose you could put a btrfs file system image on the CD/DVD and add a writable device?