I distilled this down from the Page-lists-plus plugin for WP -- a great plugin that was the sledgehammer to my mosquito.
This adds a top-level link back to the MU-Central blog (blog #1) on all sub-blogs at the end of the wp_list_pages() function. Just plop it in mu-plugins directory.
<?php
add_filter('wp_list_pages', 'page_lists_plus');
function page_lists_plus($output) {
global $wpdb;
$posts_table = $wpdb->prefix . 'posts';
if (stristr($output, '<li class="pagenav">') != FALSE) {
$output = substr_replace($output, '<li class="page_item"><a href="' . get_blogaddress_by_id( 1 ) . '">' . get_blog_details( 1 )->blogname . '</a></li></ul>', strrpos($output, '</ul>'));
} else {
$output .= '<li class="page_item"><a href="' . get_blogaddress_by_id( 1 ) . '">' . get_blog_details( 1 )->blogname . '</a></li>';
}
return $output;
}
?>