You could probably accomplish this with the Auto-filter posts by length, text, or even emoji feature.
There’s a powerful filter mechanism inside Micro.blog for automatically assigning categories based on criteria in the post. … By default, filters are applied to new posts only. To also apply the filter to all posts, there’s a new “Run Filter” button. For blogs with thousands of posts, make sure to give Micro.blog a little time to update all your posts and publish them to your blog.