I can't suggest a perfect solution but perhaps this will help?
There is a find and replace plugin that can change the links for you. There is also a plugin called "NoFollowr" which makes it easier to see what does and not not have no follow on the links and allows you to change them on the fly from the post page itself.
http://www.wesg.ca/2008/08/wordpress...-and-replacer/
http://nofollowr.com/