This Blender plugin allows you to create and then use UIs, that lay-out every bone in the rig for easy selection and toggling visibility of bone layers.
The plugin is something we developed internally for The Gateway of Realities Blender pipeline, it's still a work-in-progress and needs a lot of polish until its ready for proper public release.
Just recently it was ported for Blender 2.8+ and it has lots of bugs that need to be fixed still.
We will get there eventually, but for now only use it if you feel adventurous!
For the same reason there is no documentation written on how to use it yet, this is something we will have to do when we have more time for it.