I believe I have moved the definition of the rigid body parts and joints into the appropriate places so that rigid bodies are created off the osg root.
1. I removed the ChildOffsetTM from the Osgjoint. This was not a good solution. Instead, I just calculated what the local transform should be relative to the parent and used that. This eliminates an extra transform and simplifies the system.
2. I added a flag that tells OsgMovableItem whether it should add stuff to the parent osg part, or to the root osg node.