Great job , as usual.
This is something I intend to implement in Lightmass when I get a chance. I will take a look at your implementation and see if there’s anything I can reuse when I do so. Unfortunately I am pretty overwhelmed these days with other tasks so it will be a while. I’m planning to reuse the rays from the first bounce for subsequent bounces, so that multiple bounces effectively don’t cost extra, which is an artist expectation with Lightmass at this point.
Also, skylight needs better directionality on the sky light source, right now it’s reduced to a 3 band SH which is like an extremely low res cube map, all directionality is gone.