You can try trang to convert from one schema representation into another. It supports DTD, Relax NG (which is actually human-readable and -writable, a rare thing in the XML world!) and XML schema.
Comment on Re: XML Schemata to DTD or simplified datastructure
A schema written in any of the supported schema languages can be converted into any of the other supported schema languages, except that W3C XML Schema is supported for output only, not for input.
:-(((
I agree completely on your Relax NG comment.
So it seems I will have to do something like that myself. :-( I'll keep you all posted.