I'm pretty sure that OSGi doesn't really deal with the concept of extensions/endorsed, but rather just deals with what packages from the system classpath will be wired up to bundles. I suspect all thats required is ensuring that the org.obg.* and javax.rmi.CORBA classes are defined for the system bundle. There are a couple of yoko-specific packages in those two jars that might need to be added, but I believe those are only implementation class that would not need to exported by the system bundle.
Theoretically, it might be possible to wire these packages from a bundle, but I suspect there are JVM system classes that will need to load the same version, so this won't really work. That's the reason those jars needed to be in the endorsed dir in the first place.