Hi Dan - thank you for bringing this to my attention. It looks like the MarsEdit rich editor does support RTL layout, but it's a little obscure how to enable it:
1. Ctrl-Click in the rich editor content to bring up the context menu. 2. Select "Paragraph Direction" -> "Right to Left" from the menu.
Also, I believe that you will unfortunately have to enable it every time you want to use it. I'm not sure if there's an easier way to enable it. I'm going to look into offering these paragraph direction menu items in the main menu bar menu for MarsEdit, instead of requiring the contextual menu.