Chris,
The custom categories would actually show up underneath those top buttons. It would be similar to how the Truecolor group has sub-categories. That is the possible future expansion, however it has not yet been implemented.
So for now, your only option is to utilize the pattern name as a method of grouping or organizing them. They group by pattern name, so that multiple scales or rotation of the same pattern appear together.
I think we will definitely do some sort of reorganization and/or reordering in the future. We are open to suggestions, if you have a mockup of what you are after.
--J