Different person, but thank you for the writeup! Very interesting.
For anyone else reading: Please write more comments like this, they're one of the best parts of HN.
If anyone wants to make their own: The free e-book Ray Tracing Gems II [1] covers realtime GPU ray tracing with modern APIs and hardware acceleration, and has a chapter about spectral rendering (Chapter 42: Efficient spectral rendering on the GPU for predictive rendering).