Enabling to show private posts in parent dropdown / hierarchic CPT.
The the following code gets this done.
function show_private_posts_in_parent_dropdown( $args, $post ) {
$args['post_status'] = array( 'publish', 'private' );
return $args;
}
add_filter( 'page_attributes_dropdown_pages_args', 'show_private_posts_in_parent_dropdown', 10, 2 );
// also show private posts in the parent dropdown in quick edit
add_filter( 'quick_edit_dropdown_pages_args', 'show_private_posts_in_parent_dropdown', 10, 2 );