Christian has some info on XML and XPath features in PostgreSQL, provided via UDF. Yes, I would also love to see something similar in MySQL!
With the XSLTkit project, we provide some XSLTs that allow transoformation of XSD to e.g. SQL CREATE TABLE statements. We also plan to do the same with XUpdate to SQL ALTER TABLE transformation. This way, you can use XSLTkit to emulate a XML database on top of a RDBMS or LDAP.