New version of DocBook5 was released few months ago. Could you create ebuild for docbook-xml-dtd-5.0.0 and docbook-xml-simple-dtd-5.0.0?
I'm sincerely not sure what's the point there considering that DTDs are no longer normative for docbook 5...
Trying to cleanup the queue for sgml; a package to install the schemas for docbook5 akin to what Fedora does might enter the plans, but not for now since I really cannot pick up this myself for long, and nobody else is working on this right now.