Given a Java SE 6 client, I would like to provide a configurable back-end: either directly to a database or through a web service which connects to a centralized DB. To that end, I've created some JPA- and JAXB-annotated entity classes and a DAO interface in a POJO library like the following:

public interface MyDaoInterface {
    public MyEntity doSomething();
}

@javax.persistence.Entity
@javax.xml.bind.annotation.XmlRootElement
public class MyEntity {
   private int a;
   ....
} 

Now, I would like to have my auto-generated web service stubs implement that interface and interact with my defined entity classes, rather than the generated classes provided via the JAX-B unmarshaller. So, the client-side pseudo code would be something like

MyDaoInterface dao;
if (usingWebservice)
  dao = new WebserviceDao();
else
  dao = new JpaDao();

MyEntity e = dao.doSomething();

Is this possible with JPA, JAXB, JAXWS? Is this even advisable?

Currently we achieve this through a slow manual process of massaging code, copying generating classes, and doing other things that seem just plain wrong to me.