They call it the "Nashville Numbering System" for a reason - it's meant for use with cowboys chords for songs that are written in one key and a max of 3 chords. For flatdiminshedminorseventh chords you've got to look for the Jazz numbering system which looks a lot like Chinese Arithmetic.
Cool! In the coincidence to top all coincidi*, it just so happens that I have an advanced degree in Chinese Trigonometry, so I'm golden. I'll get right on it.
Thanks!
* = plural of coincidence