What about just applying tone curve to the luminance channel?
My take is really that raw conversion should result in a scene referred image and different kind of "looks" could be applied that image. I don't know if this is in conflict with DCP-based processing.
Current raw converters are a bit of a mixture of old film techniques and new color appearance models. White balance or tone curves do not exist in color appearance models like CIECAM02. In color science speak white balance is a simplified chromatic adpatation transform, and an RGB tone curve is an over-simplified tone reproduction operator. Color science is busy with making realistic reproductions, even emulating eyes night vision etc (meaning that you need to know exposure level too), and taking view conditions into account. To make it state-of-the-art is very complex. Documentary video seems to be the natural applications of these types of ultra-realistic reproductions.
The first part of DCP could be said to be scene referred, but the tone curve is certainly output referred. As I want DCamProf to be a profile maker also useful for the big name raw converters I need to adapt to their workflows. Personally I'd surely prefer that a profile only did the scene referred part, and the raw converter took care of all output modeling, but that's not how it works in Lightroom and Capture One for example. They expect a curve to be there, and if the profile is linear scene referred only there are no tools to add contrast to adjust the look without having color shift effects.
In photography we seem to settle at rendering D50 bright sunny scenes fairly realistically in an average viewing condition, and then accept as a bonus the extra "pop" we get when we apply the same tone reproduction and colorfulness on other conditions. I'm fine with that. Problem is that the RGB tone curve is as discussed a very bad tone reproduction operator which don't even make the sunny scenes realistic but makes color pop like crazy, and in some circumstances cause hue shift as well. Bundled profiles are semi-compensated for that, by hand I suppose for more exclusive cameras.
I'm trying to find an automated way to compensate a profile so it can be used with RGB curves without getting crazy results.
Applying contrast to the luminance channel is a good idea, but then the next question is which model of luminance should we use? Using Lab Lightness results in a desaturated look. I'm currently experimenting with using CIECAM02 lightness and that is a better perceptual model than Lab and gives better results. However, as we apply lightness contrast to simulate a brighter outdoor condition (Stevens effect) we still get a desaturated look due to the Hunt effect, to match realistically we need to add some saturation too. It's there I am now. It seems like this problem hasn't really been solved before as those that work with CIECAM02 don't work with traditional "film curves".
A color scientist would just say it can't be done without knowing all scene parameters and viewing condition parameters, but I'm looking into using that sunny D50 scene as a reference, I think if it works for that scene we'll have a suitable "neutral" look that photographers can identify with. The bright outdoor sunny scene is really the only condition that mandates increased contrast and saturation.