I think a Schema to DTD converter is your best bet for a short-term solution to your problem. I seem to remember that XML Spy could do that, although I never tried it.

Long-term, the first step to doing something like this is to create a complete spec-compliant XML Schema processor in Perl. XML::Validator::Schema is a start, but it's a long way from done. Maybe you'd like to help out?

-sam