No, I don't know anything about China. Let me know. This lens has been designed primarily by Dr. A. Suzuki.
Approximation works, of course.
I found that dispersion is not a real big problem with nonimaging devices, at least not with our lens, since the split spectrum comes from both sides of the lens, and sort of mixes on the absorber. Depends on the acceptance half angle. For a stationary CPC with, say 30deg acceptance half angle, the angles you are talking about are not too bad. And the reflectors are usually enclosed in a glass tube to avoid the dirt. Hope they publish some loss ananysis one of these days.