I think I understand you.
In that case, and for the bottom part, maybe changing the view port would do it. You would be culling the geometry plus that area of the screen wouldn't be drawn in the renderpasses of your choice.
As usual, please sombady correct me if I'm wrong.
Hmmm, viewport looks interesting for the chat bar ( entire bottom strip ).
is updateViewport an expensive call.
Wouldnt solve the plan of a map scenario though ( plan of a map can be seen in say the spirits demo - being top right ). Would a bounding volume that intersect with the cam work ??