I choose to leave it up to the composite applications to include the persistence.xml file so that they have the ability to change the database properties within it. For example, the provider, jta-data-source, dialect, show_sql, etc.
But shouldn't the core engine for all BPM services deployed in the runtime be run against same DB? Is there a purpose or real use case to allow different services deployed in the switchyard engine to run with different persistence properties?
How would I set a default configuration for the whole engine in case I wanted to?
I think this is the usual use-case, and allowing for different persistence configuration per composite should be a rare case.