@will1535 - great idea and something I've been tinkering with and plan to implement. Currently the internal color processing pipeline is really optimized for full color images. For optimal black and white processing the order of operations should be shuffled a bit so that luminance adjustments are made prior to desaturating so that the radiance curve for example actually does something useful.
Spectra should adapt to these changes dynamically, the real challenge will be to dynamically restructure the processing pipeline on the fly without compromising performance (when quickly switching between full color and black & white presets for example)

Just published an update that has a new "BW" button in the top bar. Toggling that button will output a black and white image.

All of the tools are still operating on a full color image under the hood and the black and white conversion is applied at the very end of the pipeline. This way, tools like the Radiance Curve can be used to change the greyscale parameters.

When BW is turned on, Spectra also operates exclusively in black and white.