Private page filters for WordPress. Related to ticket #8592.

wp_private_page_filters.php

/**
 * Add private/draft/future/pending pages to parent dropdown in page attributes metabox and Quick Edit
 *
 * @param array $dropdown_args
 * @param object $post (Optional)
 * @return array $dropdown_args
 */
function page_attributes_metabox_add_parents( $dropdown_args, $post = NULL ) {
	$dropdown_args['post_status'] = array('publish', 'draft', 'pending', 'future', 'private');
	return $dropdown_args;
}

add_filter( 'page_attributes_dropdown_pages_args', 'page_attributes_metabox_add_parents', 10, 2 ); 
add_filter( 'quick_edit_dropdown_pages_args', 'page_attributes_metabox_add_parents', 10);

/**
 * Add (status) to titles in page parent dropdowns
 *
 * @param string $title
 * @param object $page
 * @return string $title
 */
function page_parent_status_filter( $title, $page ) {
	$status = $page->post_status;
	if ($status != __('publish'))
		$title .= " ($status)";
	return $title;
}

add_filter( 'list_pages', 'page_parent_status_filter', 10, 2);

/**
 * Filter public page queries to include privately published ones. 
 * Filter pages metabox on menu admin screen to include all built-in statuses.
 *
 * @param object $query
 * @return object $query
 */
function private_page_query_filter($query) {
	if ( is_admin() ) {
		$screen = get_current_screen();
		if ( 'nav-menus' == $screen['base'] )
			$query->set( 'post_status', 'publish,private,future,pending,draft' );
	}
	else {
		$query->set( 'post_status', 'publish,private' );
	}	
	return $query;
}

add_filter('pre_get_posts', 'private_page_query_filter');

/**
 * Filter lists of pages to include privately published ones. 
 * This a duplicate of wp_list_pages() except we change post_status in the default args, and we return without echoing.
 *
 * @param string $output Original output of wp_list_pages(), which we will overwrite.
 * @param array|string $args Parsed arguments passed from wp_list_pages().
 * @return string HTML content.
 */
function wp_list_pages_with_private($output, $args) {
	$defaults = array( 'post_status' => 'publish,private' );  // other defaults already parsed in wp_list_pages()

	$r = wp_parse_args( $args, $defaults );
	extract( $r, EXTR_SKIP );

	$output = '';
	$current_page = 0;

	// sanitize, mostly to keep spaces out
	$r['exclude'] = preg_replace('/[^0-9,]/', '', $r['exclude']);

	// Allow plugins to filter an array of excluded pages (but don't put a nullstring into the array)
	$exclude_array = ( $r['exclude'] ) ? explode(',', $r['exclude']) : array();
	$r['exclude'] = implode( ',', apply_filters('wp_list_pages_excludes', $exclude_array) );

	// Query pages.
	$r['hierarchical'] = 0;
	$pages = get_pages($r);

	if ( !empty($pages) ) {
		if ( $r['title_li'] )
			$output .= '<li class="pagenav">' . $r['title_li'] . '<ul>';

		global $wp_query;
		if ( is_page() || is_attachment() || $wp_query->is_posts_page )
			$current_page = $wp_query->get_queried_object_id();
		$output .= walk_page_tree($pages, $r['depth'], $current_page, $r);

		if ( $r['title_li'] )
			$output .= '</ul></li>';
	}

	return $output;
}

add_filter('wp_list_pages', 'wp_list_pages_with_private', 1, 2);