I would say that being independent of the database does not mean support only a (sub)set of common features. It means being able to flexibly adapt to various implementations.
This means that (for us at least), the schema definition file(s) should contain only once what is common to all databases (the database structure), but should also provide a way to define database-specific elements.
I see four points:
(1) What is to be defined in the schema dtd (for example, should the dtd define elements for sequences and stored procedures ?).
(1.1) The xml schema used is Turbine's one. What if DdlUtils needs more or different elements ? (DdlUtils needs informations an ORM does not)
(2) How to permit the inclusion of database-specific definitions in the schma definition file(s) ?
(3) How to provide a way to get the xml schema definition file(s) from an existing database, containing the non standard elements.