DocBook v5.0 customization layer (RNC) to DTD

Hi,

I've got a fairly straightforward customization of DocBook v5b9. One of 
the major changes is adding elements from another namespace to the info 
element (extending to allow markup from Dublin Core or ODRL, for example).

I've got a valid RNC file, and have successfully validated content 
against this model. Now I need to create a DTD version of this schema 
for a particular authoring environment.

I've tried XML Hack #78 (my usual process), as well as rng2srng and
http://downloads.xmlschemata.org/relax-ng/utilities/simplification.xsl.

All of these methods produce a ton of errors (such as: sorry, combining 
definitions with combine="choice" is not supported
sorry, cannot handle this kind of "oneOrMore"
sorry, cannot handle this kind of "group"
sorry, cannot handle this kind of "mixed"
sorry, cannot handle this kind of "zeroOrMore")

I also tried using trang to convert to XSD, and it failed to complete at 
all.

Any ideas or recommended best practices for converting RNC to DTD?

One of the major reasons for moving one of my clients to DB5, is the 
ease of customization with RNC. If I can't produce a DTD or XSD, I'm 
afraid my client won't have very many choices of authoring tools since 
there is a huge lack of support for RelaxNG...

Thanks and best regards,

--Scott