One piece of input; I note a resounding silence on the subject of RDF.
RDF is a spec under active development in the W3C which is designed
to serve as a general-purpose framework for the modeling and exchange
of metadata. Possible explanations for your having bypassed it include:
1. You don't think that DTD-style schemas qualify as metadata
2. You do think of this as metadata, but don't feel that the RDF
framework is appropriate
3. Nobody brought the subject up yet
In case it's #3, now someone has. -Tim