I've worked with reproduction curves now for a while and I think I'm closing in on something that works.
Using faces is unbeatable when it comes to subjectively evaluating color, much easier to see subtle differences there than in other color ranges.
My goal is to make a tone reproduction curve where the contrast itself is a subjective/artistic matter, but hue and saturation is kept perceptually stable, ie the same as with a linear curve. That is you get to subjectively choose a contrast curve, and then color is automatically adapted to match.
The current prototype uses RGB in linear prophoto to calculate luminance change, this is to make the curve "compatible" with RGB tone curves, ie brightness/contrast becomes the same. The actual working space is CIECAM02 JCh though. Hue is kept at the original value, J (lightness) adjusted according to the curve, and C (chroma) is increased as a function of midtone contrast. Using multiplication rather than addition seems to work best for chroma increase, that is more saturated colors get more increase than less saturated colors. Some fine-tuning left to do though, and then implement in DCamProf.
It's a bit scary to do this as one have to trust one's eyes, there's nothing to verify against if the result is "correct"
RawTherapee already has this type of curve, "weighted standard" it's called (try it out if you like), it often produces good results but is a bit low on saturation easily noted on skin tones. In the RT team we're looking at adding a new curve with better properties. Not sure my curve will do then as it's a bit slow, at least in my current implementation. We'll see...