Adding some thoughts from discussions via ML and IRC.

It would be nice if there was a way for providing custom generator annotations such as @CreationTimestamp. Such a generator annotation type might be defined like this:

@Generated(
    when = INSERT,
    generator = MyCreationTimestampGenerator.class
}
public @interface MyCreationTimestamp {
    TemporalType type();
    String source default "vm";
}

Here the @Generated annotation is used to mark @MyCreationTimestamp as custom generator annotation type, specifying the generator type and when to invoke it. A custom generator annotation may define additional attributes as suited per its requirements.

The generator of such an annotation would implement a specialized contract which receives the annotation instance upon initalization:

public interface CustomDynamicValueGenerator<V, A extends Annotation> extends DynamicValueGenerator<V> {
    void initialize(A generatorAnnotation);
}

public class MyCreationTimestampGenerator implements DynamicValueGenerator<String, MyCreationTimestamp> {

    TemporalType type;

    void initialize(MyCreationTimestamp annotation) {
        this.type = annotation.type();
    }

    String generateValue(Session session, Object owner) {
        //...
    }
}

Thinking about it some more, when given on a custom generator annotation type, @Generated#generator() should only accept a CustomDynamicValueGenerator type, so having a special annotation might be the better idea.

Optionally it might also make sense to allow for specifying the time of execution for custom generators:

@Modifier(when={INSERT, UPDATE})
private String lastModifier;

Unfortunately one can't define a contract for annotation types by means of an interface, so one could only declare a naming convention for this method.