You have access to luminance for each colour after conversion to B&W in LR, and you can see the effect in real time.
If I can it is not evident in the latest (well, as of 3 months ago) LR subscription on my older MacBook. Maybe I'm missing something. I'll feel very stupid if I have. Again, I'm talking about accessing luminance in the color tools palette along with with hue and saturation.
I remain mystified by what transformation you are applying by increasing the saturation in one colour before forcing the total saturation to zero: it looks to me by multiplying by whatever number you want followed by multiplication by zero.
"I think what we have here is a failure to communicate." And it well may be my fault, if so I apologize. First, I never convert to B&W by forcing saturation to zero. Second, let's step back and look at my workflow again to see what I'm trying to say.
Consider an image with a bright blue sky. I want the sky as near black as possible in the B&W conversion. So I do all the typical steps up to B&W conversion. In CO or Aperture I use the full B&W conversion panel. Same in LR. I do not desaturate anything. I do not use a preset. Using the color sliders in the B&W conversion panel I slide the blue and sometimes cyan channel to the left. This darkens the blue but maybe not as much as I want OR
it gets as dark as I want but there is banding on close inspection of the sky. So I back off a little until all the banding is gone then I go into the color management panel...........all the way out of the B&W conversion panel. In LR this would be the HSL grouping in the color tools. Move the sliders...nothing happens...at least for me. In CO or Aperture moving the luminance slider for blue immediately has a profound affect and is usually all that I need. However, the saturation slider will also allow some additional darkening. In CO you have to be delicate with the saturation as it will produce very dark corners in a blue sky across the frame....a sort of vignette. Not quite as heavy handed in Aperture. Changing the hue can yield a tiny change but not much.
Again. all this fiddling is just for that last tiny little tonal adjustment. Maybe I'm trying to be unnecessarily precise.....but in my brief experience here at LuLa there seems no end to the pursuit of of that perfect tone.
Why does this work? I can't say. But I'm assuming in the B&W conversion panel the blue slider has its effect on a certain range of blue HSL. Changing that range will have additional effects on that adjustment. That's my best guess.