Actually I just figured this one out on my own. This is already possible using two more instances of color balance RGB, one for the blacks and one for the whites. You have to make the shadows/highlights falloff very steep, and then you control the mask through the mask middle gray fulcrum.
It works better in the shadows than in the highlights, though, since it allows me to go as far as to actually not select any shadows. That doesn’t happen in the highlights, but that might be a bug on my version.