Custom Post Type as Page parent

So there are a million snippets for how to get pages as parents of custom post types.

However, the reverse seems to be questionable. One would think, since everything in WordPress is technically a "post", this would be trivial. However, it is not.

Thus far, I have:

add_filter( 'page_attributes_dropdown_pages_args',
  function( $dropdown_args, $post = null ) {
    $dropdown_args['post_type'] = 'portal';
    return $dropdown_args;
  } );

And sure enough, pages now allow for me to select portal-type posts as parents. The URLs do seem to register correctly within wordpress, though they all 404.

How can I get wordpress to understand this hierarchy and load the pages that have posts as parents?