per pixel culling

I would like to add per pixel culling (shader) to the engine and editor, what would be the simplest way to do?

Or even a portal vis area system.

Thank you

Edit: Trying out the shader replacement demo’s, I’m on a G5 mac with an ATi 9600, the shaders don’t seem to work… help please, I have the latest drivers so they should support OGL 2.0.

Seems likes the shader replacement demos don’t seem to work in the demo version, once I upgraded they seem to work now.