Currently, it is not possible to add a relationship as a field in a metabox. But it's a good idea, I will inform the developer team to consider adding it to the to-do list for the future development of the plugin.
Is this feature of adding custom fields to a relationship, or adding relationships fields to a meta box, on the roadmap yet? It would really make displaying related content advanced. I could then, for example, also be able to display the reason for the connection.