Gail S. answered 07/08/19
Adobe Certified Lightroom Tutor
Hi! This is a good question. You're right in that it cannot be done directly. However, if you keep the file in RGB and desaturate in the HSL panel (move the S to 0 for all colors), you can then go back to the Tone Curve and apply a curve to each channel, R, G, and B separately. I find the Lightroom Tone Curve very sensitive, so you need to take care when applying the curves. It's an interesting way to get unusual tones for a monotone image.
By the way, I never convert to black and white by clicking on Black & White in the Basic Panel of the Develop module . I always use the HSL panel with 0% saturation. This allows me to affect the luminance (brightness) of each color in the original separately. Gives me much more control over the black and white 'look'. Lowering the luminance of blue, for instance, would darken the sky. If you have reds and greens that don't 'separate' as grays, the luminance slider can handle that quite nicely as well.