Seeding of non-artist entities including relationships had been implemented in MBS-7913 and the processing was moved to the server side in MBS-11985.
While this allows to seed almost everything, including the credited-as field of relationship attributes (via rels.X.attributes.Y.credited_as), it is not yet possible to seed the credited-as field of the relationship's target entity (or the source entity, which is currently not even possible via the UI).

The current parser code sets both entity0_credit and entity1_credit to an empty value unconditionally. It would be great to be able to seed this via rels.X.target_credit or rels.X.entity0_credit / rels.X.entity1_credit (one of them belongs to the target entity, depending on the relationship type).