This is an interesting idea and a very compelling image -- basically grouping hotbar icons into named categories. You could create an image of the tab areas and drop it into a custom window like you suggest, but every time your spell layout changes you'll have to make a new image. Hotbars can be edited to have a margin area above the icons, but the problem is that all of your hotbars would have to have them. It would be much tougher to pick just a couple of hotbars to add a "titlebar" area since they are all clones from just one definition file.
I guess a custom window could be written so you could add some text into boxes with variable widths to approximate what is shown in your image. The custom window would float above (or below) the hotbars in your example.