I needed to implement a subset of these features in a different plugin I’m working on, so decided to spin them out as a separate plugin, as even in this early stage it may prove useful to others (and it makes building upon them, and implementing this whole spec, easier in future):
https://github.com/mozilla/discourse-group-category-notification
The readme explains a lot of the caveats, the biggest of which is probably the lack of a UI.