It actually does work, the only thing its missing is the sub categories listed in the side bar menu in some sort of a hierarchial format.
If you know the category number, you can enter it in the url
http://www.yoursitecom/graffiti-admin/site-options/custom-fields/?category=12 <-- change the 12 to whatever category number you want.
Personally, I wouldnt do this because if someone else manages the site later and doesnt know this, they wont be able to add/remove custom fields, so use at your own risk.
Submit a feature request and we will look into adding all categories to the list.