Get a multiple value listing of related node IDs using Relation module and Views node ID field
A work around which could come in mind to an experienced Drupal site builder is to use Views module to get more flexibility in showing other fields of related entities listed instead of titles, by using the Views relationship advanced settings in connection with an existing field of related items like node ID.

Sadly the multiple value field handler of Views only appears on the Relation endpoint field itself, added directly in your Views view, but not in the node ID field by using the proper Views relationship to the relation &lt;-> for this purpose.

The expected behavior is, that you are able to activate and modify multiple values of the related node ID field now in the node ID field settings like you did in the endpoints field before. But sadly that's not the case. You will always get the first related node ID or many duplicate records depending on how much related items you have with each related node ID per Views view row.