This is a great idea
I am using NValt for notetaking and I have pointed my NValt folder to a folder inside of a EagleFiler library.
For Markdown preview, I’m using the excellent Marked2 app by Brett Terpstra.
But having markdown rendering straight in the document viewer would be gold!
Preferably styled like Byword/Ulysses etc i.e. syntax highlighted but not transformed/complied completely.