How do I get_the_content in a custom walker?

For anyone who might be looking for the same thing, this is what I did in the end:

I abandoned the Custom Walker idea and used wp_get_nav_menu_items to get a list of post IDs from my custom menu, from this tutorial on Digging into WordPress: http://digwp.com/2011/11/html-formatting-custom-menus/

In my functions file, I created this function:

function salamander_fetch_advent_posts() {
 global $post_list;
 $menu_name="advent-calendar"; // specify custom menu slug
 if ( ($locations = get_nav_menu_locations() ) && isset( $locations[$menu_name] ) ) {
    $menu = wp_get_nav_menu_object($locations[$menu_name]);
    $menu_items = wp_get_nav_menu_items($menu->term_id);

    $post_list = array();
    foreach ((array) $menu_items as $key => $menu_item) {
        $post_id = $menu_item->object_id;
        $post_list[] = $post_id;
    }
  }
  return $post_list ;
}

And then in my theme template file, I passed $post_list to a new query, like so:

global $post_list;
if (function_exists( salamander_fetch_advent_posts() )) {
    salamander_fetch_advent_posts();
}

// The Query
$the_query = new WP_Query( array(
    'post__in' => $post_list,
    'post_type' => 'any',
    'posts_per_page' => -1
    ) ) ;

// The Loop
while ( $the_query->have_posts() ) : $the_query->the_post();
    echo '<li>';
    the_title();
    echo '</li>';
endwhile;

// Reset Post Data
wp_reset_postdata();