Ah, I had no idea that extension was included in the markdown standard library.
I have no idea what that extension does, in fact, I dont see how it could possibly work, since all markdown extensions do is change the html generated from the input txt and html has no standard for metadata. Indeed, reading
https://pythonhosted.org/Markdown/ex...meta_data.html the page says it does not do anything.
So if you want support for metadata in markdown, you'd have to modify the calibre txt input plugin to make use of the metadata read by that extension.