Thanks. Not sure how the new category idea would work. I guess admins would have to manually move popular posts into that category to be zapiered to FB. But it does have potential for having a special FB category where posts are explicitly copied to FB.
I did think of creating a dummy user, locking it to Trust 0, and not logging into it so discourse would mail it a daily digest summary and using Zapier to report that on Facebook groups. The only downside is that it includes a summary since the dummy account last logged on, but otherwise it’s close to what I want - a summary of popular publicly visible (trust 0) posts.