Interesting thought, but I'm not convinced. Constructing a robust BSP tree is very expensive, because you need exact rational arithmetic and the number of bits

Interesting thought, but I'm not convinced. Constructing a robust BSP tree is very expensive, because you need exact rational arithmetic and the number of bits grows quite large. You wouldn't want to do that at runtime, so you could only use this for static geometry. If most of your geometry is opaque, you can already achieve the same reduction in lighting complexity with a Z pre-pass.