I'm going to look at this. I've recently finished working on something that uses php to publish posts to Wordpress. I figured I could probably use x-callback-urls for this. It might be little rough but I don't think it would be hard.
I second/third/fourth publishing to Medium. Markdown > Medium would be INCREDIBLE. Especially if it had image support. This is one strength of iA Writer that, would Bear develop it, would make it easier to make a full switch.
If this feature becomes available, I would love to know about it!
+1 for the publish to WordPress. I looked for it in the "Share to" list, but it wasn't listed under my extensions. I don't even know if that's possible.