Converting Relax-NG to Schema and DTD

Hello all,

I have a question: How can I get a DTD from a RelaxNG schema, if Trang 
refuses to generate one?


A somewhat longer version of the same question is:

I work in the OASIS Open Office Technical Committee, which works on an 
standardized XML file format for office applications, which will likely 
be supported by at least OpenOffice.org, KOffice and StarOffice. The TC 
uses RelaxNG as a working language, which means that the formal part of 
the specification is all done in RelaxNG. But we would like to ship 
(non-normative) W3C XML Schema as well as DTDs, since many users cannot 
freely choose the schema language for the work and hence require either 
W3C XML Schema or DTDs.

So... We would of course like to generate .xsd and .dtd files 
automatically. Trang does a very nice job on the former: The .xsd is 
nicely readable, and mostly validates correctly. The remaining problems 
can be fixed easily.

However, we couldn't generate DTDs. Trang refuses to generate them. 
We've tried generating DTDs from the generated .xsd files with XML-Spy, 
but the resulting DTD was not even valid. Does anyone else have 
experience with converting RelaxNG to DTDs? What kind of tools work well 
for this task?

Thanks!

Sincerely,
Daniel Vogelheim