One thing I realize now is that software that supports XMLRPC (or better Atom) posting can support whatever client-side rendering I want (but not editing, which is OK). So I could compose in reST, then have something that renders to to HTML and uploads it as a post. Which is good enough for me (though I also hope that at least something reasonable is available for commenters -- seeing how hard it is to talk about code in the Radio commenting environments shows how difficult that can make things).
I wrote a little script to post comments to my wordpress blog from emacs (or whatever editor you might use), except that I used textile as a text formatter. I imagine there's a python module for rest, so you should be able to use it with it. Here's the URL: http://aida.homelinux.net:3050/scripts/file/wordpress/write-blog.py