Yes, this was meant to mimic the PHPCR ability to refer routes to any other Document (required by routing to have any entity served at controller callback). I was actually using an helper to set the relation (something like get_class($entity) + $entity->getId()) so the class path does not contain colon, but there's room for suggestions/other solution ofc.
There could be an enhancer per entity, that would only require more configuration options. The need to have such a 'ContentRepository' for orm is related to the route enhancement process, so maybe that should be more explicit.