Exporting to an RTF document directly is not actually supported by MultiMarkdown. Composer for Mac OS accomplishes this by first converting your document to HTML. It then uses the built in functionality of Mac OS X to convert HTML to RTF. This is basically the same approach that was used in the original version of Multimarkdown by using the
If you want to customize the appearance, you can embed CSS information in the metadata, and this will be incorporated.
Please keep in mind that RTF is not an officially supported output format for Multimarkdown, so this is simply a workaround to include a version of this functionality in the Composer application.
A more reliable, but less direct, approach is to convert to the Flat OpenDocument format first, and then to use Libre Office to convert to RTF (or other format such as Microsoft Word).
Note:Composer for iOS does not (currently) support converting directly to RTF, but it does convert to OpenDocument.[an error occurred while processing this directive]