Oh I like that idea as an addition to my plan.
My primary goal is for the facebook group to regularly broadcast the existence of a vibrant discourse forum (to use @codinghorror’s term) off facebook. The digest mechanism can be leveraged to do that because it collects a summary of the most popular things that have happened in the past day by using zapier to repost digest mails into the facebook group.
But I can also add the ability for a user making a post to request the post to be copied to the facebook group by tagging it with a specific magical tag. And then the .rss feed of that tag can be used to add posts to the facebook group.