You could probably spoof a directory browsing feature by giving each directory its own genre or year or something. You could use MP3 Tag Tools to update the files on the Sansa itself so you're not messing with your originals.
I find Genre tagging to be all over the place, to the extent that it's pretty useless (is something Rock? Alternative? Alternative & Punk?). Also, is it that important to know the year that something was made? Do you want to play only songs from 1974?
So, if you find those tags as useless as I do, you can use them as placeholders for directories.
Just a thought.