Until thats done, i think that could be done with an automoderator. E.g. iff a post title begins with [ONLYLOCAL] then trawl the comments every n minutes and remove all comments from nonlocal users. Maybe it would fall apart at scale? But im not sure. It would certainly be more performant as part of the software itself, but i do think its possible with an automod.
I love the email workflow: open GNUS, hit C-o on the email, and boom i have a buffer holding a patch that can be applied with git am!