Post suggestions regarding RVGL.
The current behaviour of visiboxes could be changed. Instead of hiding entire cubes it could hide single faces (tris and quads).
This is how it already works - partially intersecting cubes have per-poly visibox tests. In fact Re-Volt featured a -cubevisi parameter to disable per-poly visi tests (maybe they were slow on 1999 computers). The parameter still exists in RVGL.