Nested groups aren't supported at the moment at least.
Errante's Filter by Column add-in may be useful if the main aim is to drill down to a particular level. Filter by Column : Powerful filtering/searching tool for all columns
Generating nested collections (or folders of shortcuts or symlinks) to the real files is another option.
The only other way I can think of would be to write a VFS plugin, to map the real filesystem to a different structure. That would let you get exactly what you want, but would be a lot of work as VFS plugins are on the more complex side of things (at least at the moment).