This walker for WordPress gives you the ability to only show the items that are selected

walker-page-parent-only.php

<?php

/**
 * Create HTML list of pages.
 *
 * @package WordPress
 * @since 2.1.0
 * @uses Walker
 */
class Walker_Page_Parent_Only extends Walker {
	private $show_lvl = true;

	/**
	 * @see Walker::$tree_type
	 * @since 2.1.0
	 * @var string
	 */
	var $tree_type = 'page';

	/**
	 * @see Walker::$db_fields
	 * @since 2.1.0
	 * @todo Decouple this.
	 * @var array
	 */
	var $db_fields = array ('parent' => 'post_parent', 'id' => 'ID');

	/**
	 * @see Walker::start_lvl()
	 * @since 2.1.0
	 *
	 * @param string $output Passed by reference. Used to append additional content.
	 * @param int $depth Depth of page. Used for padding.
	 * @param array $args
	 */
	function start_lvl( &$output, $depth = 0, $args = array() ) {
		if( ! $this->show_lvl )
			return;

		$indent = str_repeat("\t", $depth);
		$output .= "\n$indent<ul class='children'>\n";
	}

	/**
	 * @see Walker::end_lvl()
	 * @since 2.1.0
	 *
	 * @param string $output Passed by reference. Used to append additional content.
	 * @param int $depth Depth of page. Used for padding.
	 * @param array $args
	 */
	function end_lvl( &$output, $depth = 0, $args = array() ) {
		if( ! $this->show_lvl )
			return;

		$indent = str_repeat("\t", $depth);
		$output .= "$indent</ul>\n";
	}

	/**
	 * @see Walker::start_el()
	 * @since 2.1.0
	 *
	 * @param string $output Passed by reference. Used to append additional content.
	 * @param object $page Page data object.
	 * @param int $depth Depth of page. Used for padding.
	 * @param int $current_page Page ID.
	 * @param array $args
	 */
	function start_el( &$output, $page, $depth, $args, $current_page = 0 ) {
		if ( $depth )
			$indent = str_repeat("\t", $depth);
		else
			$indent = '';

		extract($args, EXTR_SKIP);

		if( isset( $filter_selected ) && $filter_selected ) {
			if( $depth == 0 ) {
				$this->show_lvl = true;
			}

			if( ! $this->show_lvl )
				return;
		}

		$css_class = array('page_item', 'page-item-'.$page->ID);
		if ( !empty($current_page) ) {
			$_current_page = get_post( $current_page );
			if ( in_array( $page->ID, $_current_page->ancestors ) )
				$css_class[] = 'current_page_ancestor';
			if ( $page->ID == $current_page )
				$css_class[] = 'current_page_item';
			elseif ( $_current_page && $page->ID == $_current_page->post_parent )
				$css_class[] = 'current_page_parent';
			else if ( ! in_array( $page->ID, $_current_page->ancestors ) )
				$do_filter_selected = true;
		} elseif ( $page->ID == get_option('page_for_posts') ) {
			$css_class[] = 'current_page_parent';
		}
		else {
			$do_filter_selected = true;
		}

		if ( isset( $filter_selected, $do_filter_selected ) && $filter_selected ) {
			if( $depth == 0 )
				$this->show_lvl = false;
		}

		$css_class = implode( ' ', apply_filters( 'page_css_class', $css_class, $page, $depth, $args, $current_page ) );

		$output .= $indent . '<li class="' . $css_class . '"><a href="' . get_permalink($page->ID) . '">' . $link_before . apply_filters( 'the_title', $page->post_title, $page->ID ) . $link_after . '</a>';

		if ( !empty($show_date) ) {
			if ( 'modified' == $show_date )
				$time = $page->post_modified;
			else
				$time = $page->post_date;

			$output .= " " . mysql2date($date_format, $time);
		}
	}

	/**
	 * @see Walker::end_el()
	 * @since 2.1.0
	 *
	 * @param string $output Passed by reference. Used to append additional content.
	 * @param object $page Page data object. Not used.
	 * @param int $depth Depth of page. Not Used.
	 * @param array $args
	 */
	function end_el( &$output, $page, $depth = 0, $args = array() ) {
		if( ! $this->show_lvl )
			return;

		$output .= "</li>\n";
	}
}

Forked this for future reference. Thanks again Marko! :)