Just heard from Brent that the next version of NetNewsWire will support the source:markdown element.
I wrote it up on my daveverse site, which I edit with WordLand, which means it has a special feed that contains source:markdown elements.
You can be one of the first people with markdown-support in your feed, help the bootstrap. I'm pretty sure it's going to work, at this point.
If you have questions, you can post a comment here.