<a target=”_blank” href=”http://blahblah.com”>new link</a>
you would use
<a rel=”external” href=”http://blahblah.com”>new link</a>
In my opinion, there comes a time when compliancy gives way to over enthusiasm and we might be reaching the limit in this case. In order to use a well documented (albeit arcane) HTML feature and keep it compliant with XHTML, one has to go around the block, add slow code and make things more complicated than they have to be. *Shrug*
If you know of a better method, please leave a comment so I can add yours to the list.