So for us to have occlusion at the triangle level it really does need to be built into the Blitz rendering pipeline. Correct me if I'm wrong, but it looks like blitz just throws the entities ( not Terrains or BSPs ) into the render engine which only culls away complete entities outside the view frustum, I'm sure that Blitz leaves Direct3D to hide the back facing polys ( hence you dont get a true TrisRendered() ).
Another thing to mention is that say for example we go for an 'entity culled occlusion' which most are getting to work, we mustn't forget that the game still goes on in 'culled sections' of the game so if we are using HideEntity then that entity is no longer involved in the collision engine - EntityAlpha will still be involved in collisions.
Another thing to mention is that say for example we go for an 'entity culled occlusion' which most are getting to work, we mustn't forget that the game still goes on in 'culled sections' of the game so if we are using HideEntity then that entity is no longer involved in the collision engine - EntityAlpha will still be involved in collisions.