I like the idea of an annotation to define which registry to target, but I'd not use it for discovery as I think the nice point of having an XML configured service is that it's explicitly listing what will happen. (automatic registrations would be a problem in case of conflicts or unwanted dependencies on the classpath).

@BootService seems a nice example, but I'm not sure if you can define all of them. I still expect to be able to add additional "layers", so that for example Search registry could use have a lower layer attached to the chain of services. So for example we could have some services which "shadow" the ORM service when it's requested from the lower layer but without affecting the upper layer. Or simply because we have multiple differently configured leafs which want to share the same ORM instance.