Updated: the download link on this page now goes to the fourth revision, more information here.
After my experiment last night, and finding DivineDominion’s already-awesome fork of Notational Velocity, I decided to port my changes over to his fork and work from there. So here’s the current version of the final product, with the intial visual changes, CSS styling, and quick access to HTML source for copy/paste. The preview is now in a HUD instead of a third pane, which I’m finding I like better. There’s more room for editing and the HUD hides on deactivation, so it’s not in the way if you switch applications.
It turns out that there are still a few changes I’d like to make. First and foremost, I want to get it set up so that the MultiMarkdown can render fragments, as I don’t think you ever need to copy a full XHTML document out of the preview, and there’s a “Save HTML” button for that purpose. I’d also like prettier scrollbars, so I’m working on that. I’d also like to add some basic Markdown editing features like auto-pairing and wrapping, but my Obj-C string handling skills are a bit lacking. I’ll keep you posted.
And yes, I’ll post the code on GitHub as a fork when I get a chance.