Indigo Renderer Forum

I found a nice paper about path integration which is supposed to lead to same results as MC with much less computational efford.

However, this is just an approximation so I'm not sure if it even fits into your philosophy about indigo.

Anyway, here it is:
http://www.cs.utah.edu/vissim/papers/PathIntegration/

The same guys also published a paper about realistic daylight, meaning a sun/sky combination:

http://www.cs.utah.edu/vissim/papers/sunsky/