One of the users from WordPress MU migrated two plugins for his site, and he was nice enough to send them over so we could make them available to everyone. These plugins should work on both WordPress for Joomla! and WordPress MU for Joomla!. I have not tested them but I briefly looked over the code and it looks good. They are now available for download.
The Powerpress plugin gives Joomla! the full blogging and podcasting suite.