It’s possible we’ll support them in the future. We’ve disabled them for now, because they break if you include footnotes and use the Excerpt / “Read more” feature in any given post.
If anyone is interested in working on a fix, that’s always welcome! Right now we simply truncate the post and render the markdown on the initial excerpt. To fix this, we’ll need to render the markdown based on the entire post, truncate it at the <!--more--> tag, and make sure the footnotes option in the writeas/saturday library is enabled.