Have you considered making a Quicksilver Plugin for MarsEdit? I currently have a trigger for the Post with MarsEdit bookmarklet but I'd like a little more.
How about a New Post action for MarsEdit (that is, only available when MarsEdit is in the first pane)?
I'd like to be able to right-arrow into MarsEdit to see the weblog list (including drafts) and then right arrow again to see a list of posts.
For a given post I'd like actions to edit the article or open it in the browser or copy the URL to the clipboard. This last one would be useful for referencing an older post when writing a new one.