Hi Ryan,
Each relationship meta box has an ID: {$relationship_id}_relationships_from
and {$relationship_id}_relationships_to
. You can use these IDs to get the meta box registry information. For example
$meta_box_registry = rwmb_get_registry( 'meta_box' );
$registry = $meta_box_registry->get('{$relationship_id}_relationships_from');
Get more details here https://docs.metabox.io/rwmb-get-registry/