Would it be possible to add a filter to only Display items if the Overlay for the item is selected.
Basically, add a new option: where you could either double click the overlay (or the eye)
So, in this example, the EYE is open for all the Overlays except CoreSandbox, so, if there were any CoreSandbox items in the classes list, they would not display.
Or, we could turn off vision for all overlay items except CoreSandbox, which would gives us only the CoreSandbox classes.
Just a thought.