1.
James, sorry for the misunderstanding.

2.
The list of XML languages for describing OO structures in
http://www.ruby-talk.com/34629
is not comprehensive.

3.
Dave, are you interested in using an existing language for RDoc's XML 
output, for interoperability?
Perhaps other tools can read it to generate documentation or graphs.
If you develop your own:
Are you planning on creating some schema(DTD/etc.) for it?
One could lookup stuff in the schema while writing an XSLT or other 
processing programs.

Tobi

-- 

* peace&love.
* http://www.pinkjuice.com/