Been asked this a few times and needed to figure it out myself, so I put together a post on how to host custom HTTP help pages for your WCF services:
http://blog.iserviceoriented.com/index.php/2010/05/04/building-custom-http-help-pages-with-wcf/
A little help from the WCF team to open up some of the internal classes would make it more straightforward... until them, it takes a bit of hacking and black magic.