How do I share configuration between custom admin form and node type edit form?
So the bottom line appears to be that Third Party Settings is a great way to handle content type settings without really caring which content type you're dealing with, but there doesn't seem to be a way to get this working with a custom admin settings form for the reason described in the question. 

If I didn't want to have a separate configuration form that handled the content type settings for all content types on one form, I would definitely have used Third Party Settings. However, since I had this extra requirement, I wound up fixing whatever broke along the journey from D8 beta to stable release. 

Now the node type gets included with each of the desired settings using the following code:

function mymodule_form_node_type_form_builder($entity_type, NodeTypeInterface $type, &amp;$form, FormStateInterface $form_state) {
  $node_type = $type-&gt;get('type');
  $config = \Drupal::getContainer()-&gt;get('config.factory')-&gt;getEditable('mymodule.settings');
  $config-&gt;set('my_setting_'.$node_type, $form_state-&gt;getValue('my_setting'));
  $config-&gt;save();
}