To suit my own purposes, I’ve hacked the plugin like so. This is good enough for me, but doesn’t allow for user choice.
I replaced:
if ($instance['show_topmost']) {
wp_list_pages( 'title_li=&include='.$top_page.$link_addons );
}
with
if ($instance['show_topmost']) {
if (is_page($top_page)) {
echo '<li class="page_item page-item-'.$top_page.' current_page_item"><a href="' . get_permalink($top_page) . '">Overview</a></li>';
} else {
echo '<li class="page_item page-item-'.$top_page.'"><a href="' . get_permalink($top_page) . '">Overview</a></li>';
}
}