I may be misunderstanding your ask, but writing permissions for groups is definable via groups as per
Or are you referring to setting up a list of which groups another group is allowed to mention via that interface?
This would break the more normal use case though – e.g. I belong to lots of groups here and if a post was published to all categories that are accessible to groups I belonged to, it’s going to be all over the forum.
We are reconsidering the way that groups and categories work, but this seems like a pretty bespoke requirement.