I just got word from Brent that the next release of NetNewsWire will have support for the source:markdown element.

This will help the transition to using Markdown as a fully-supported format for RSS. 

NNW is the first mainstream product to support it, as far as I know.

This site has an RSS feed that contains source:markdown elements.

Every WordLand-written WordPress post has one of these feeds, so there is an official way to publish content that NNW will display via Markdown. Just sign on to WordLand and start writing. There is a link to the special RSS feed in the popup menu in the editor.

I started a thread on GitHub to answer questions, etc.