Is there a hook that could manipulate the list of field items before the field is built?
What about hook_entity_prepare_form()? This simple example loaded an extra item into the form.
function mymodule_entity_prepare_form($entity, $operation, $form_state){
  if ($entity-&gt;bundle() == 'domain_groups') {
    $items = $entity-&gt;get('field_topic_access')-&gt;getValue();
    //just a hardcoded item for test purposes
    $demo_item=['topic_tid'=&gt;'74','available_to_group'=&gt;'1', 'publish_to_group'=&gt;'0'];
    $items[] = $demo_item;
    $entity-&gt;set('field_topic_access', $items);
  }
}

There may be a more elegant solution but I could use this hook to show my inherited items and hook_entity_presave()  to remove inherited values before saving.