I brought this up a while ago (darktable's filmic FAQ - #107 by europlatus) and have always liked the idea. ART has a false colour mask and I really liked it. It’s also quite standard in the video industry I believe, so it’s not a crazy idea.
The zone system module provided similar functionality, but with that being deprecated, there’s now no similar feature at all.
Could that visual feature from the zone system module be somehow ported to a mask so that we can get the functionality back without the full module?