How hard to implement a data layer to use a RDBMS as a storage? e.g. Oracle, Postgre
...probably similar to implementing the BaseX storage as a data layer for Oracle ;) The BaseX XQuery Processor is tightly coupled with the database backend; otherwise, performance of XPath requests would decrease a lot. If you want to store data in an RDBMS, what about using the XQuery SQL Module [1]?
Best, Christian