How to create a non-destructive luminosity/b&w mask for (de)saturation?

Question

Someone asked me this question: CG renders (created in Modo, Max, Blender, Maya, Vray, Octane, etc) can be made to look more realistic by desaturating the colours in the highlights. It was also requested that this move would be done in 32bpc as well (something that is often required in a CG compositing workflow).
This is easy enough to do: create a selection based on luminosity (or a black and white version of the image), and use that as a layer mask in the Hue/Saturation adjustment layer, and turn down the saturation.
However, this is a destructive action: suppose we want to import an external file as a smart object, and then we need the option to have the file automatically update with any change we make to the original artwork.
In that case we would have to recreate that luminosity mask again and again as well.
Ideally, an update of the external file will result in an automatic update of the entire layer stack.
Now, in Photoline this is easy to do:
Since layer masks behave like regular layers in PL, and layers can be virtually cloned (not unlike smart objects), it is a mere case of cloning the original external file layer twice, and adding adjustment layers. Finally, that one instanced layer is used as a layer mask for the adjustment layer that controls the saturation. Done.
Replacing or editing the original source will then automatically update and cascade the changes through the entire layer stack.
This is a very handy technique to have! Since I teach Photoshop classes, I thought this would also be handy to know how to do in Photoshop, and I tried several methods (clipping masks included), but I cannot seem to achieve the same non-destructive result. Clipping masks do not work with adjustment layers. Groups cannot be used as clipping masks. Layer masks cannot reference a smart object.
I have a feeling there ought to be a reasonably straightforward method to achieve this, but how?
Would anyone have a suggestion how to solve this in Photoshop in a non-destructive way?

Answer 1

csuebele wrote:
I'm still not seeing it. Your example does not have a pixel based layer clipped to the group which is causing my problem. I hope I can explain this. See below. the layers in the group create the "mask". In this case a b&w adjustment layer was added to control how the "mask" is converted to b&w. You can see what these layers look like in the mid section top section of the image. any color showing though is from the very bottom layer, as the group has a blend if on it allowing the shadows to come through. The top image has a curve layer with no adjustment, while the bottom image has a curve layer to clip more of the shadows. You can see this change in the upper right corner of both images. The curves are allowing more of the image to be clipped in the area is masked out on layer "Layer 0 copy 1", as more of the color of the base layer is showing though. However, that b&w areas are suppose to be just the mask and you should see the b&w, just color as seen in the bottom image. However, the curves are not changing the transparency in the areas that have the layer to return the color.
I don’t get it – what good is the pixel Layer ("Layer 0 copy 2") clipped to the Group supposed to do? Don’t you want to use the Mask on an Adjustment?
Edit: Maybe this can help clarify how I suppose the issue would be approached.