The best way to do it would be if you had access to the draw order, then you could draw the cube last and just render it to a target the same size as your backbuffer, and use the same zbuffer, with zenable=on and just clear everything on your new target to black.

So bascially to do it currently, you will need a simple dll, that uses a render event triggered by render_entities .. then just do the steps i mentioned.