Sure, but for the sake of argument lets say the value in my tree applies to all areas not covered by a specific child node. I have never used that in a 3D tree, but I have in a 2D tree.
That brings us back to a plausible struct that is forced to use an enum to get the indirect feature. It really isn't the end of the world, if the error message directed one to one of the existing solutions I would say this is more like a paper cut.