I've seen mention on the internet of python libraries that convert to a real .rtf file. One is pyRTF. I imagine it would require Pythonista, not Editorial (better support for external libraries), and I imagine it would take some sweat to get it running.
There's an iOS app called textilus that supports markdown and outputs rtf. It would be simple to have an editorial workflow to put the current open markdown file on the clipboard, then you would manually switch to textilus, paste the markdown into an empty document and output it as rtf. Caution: I haven't used textilus or done this, but it seems in the realms of the possible.