Dynamically link to the latest post or simulate request of specific post in page template

You can filter wp_nav_menu_objects and add a new item. Here is a simple plugin doing that:

<?php # -*- coding: utf-8 -*-
/**
 * Plugin Name: Latest Post In Menu
 * Description: Append a link to the latest post to all nav menus called with the argument <code>'add_latest_post' => TRUE</code>.
 * Plugin URI:  http://wordpress.stackexchange.com/q/59892/73
 * Version:     2012.07
 * Author:      Thomas Scholz
 * Author URI:  http://toscho.de
 * License:     MIT
 * License URI: http://www.opensource.org/licenses/mit-license.php
 */

add_filter( 'wp_nav_menu_objects', 'wpse_59892_latest_post_in_nav_menu', 10, 2 );

/**
 * Add a link to the latest post to the nav menu.
 *
 * The nav menu has to be called with 'add_latest_post' => TRUE.
 * Example:
 * wp_nav_menu(
 *  array(
 *      'theme_location' => 'primary',
 *      'add_latest_post' => TRUE
 *  )
 * );
 *
 * @wp-hook wp_nav_menu_objects
 * @param   array $sorted_menu_items Existing menu items
 * @param   object $args Nav menu arguments as object.
 * @return  array
 */
function wpse_59892_latest_post_in_nav_menu( $sorted_menu_items, $args )
{
    if ( ! isset ( $args->add_latest_post )                    // argument set?
        or ! $args->add_latest_post                           // argument TRUE?
        or ! $latest = get_posts( array ( 'numberposts' => 1 ) ) // post found?
    )
    {
        return $sorted_menu_items;
    }

    // Uncomment the following line to see what you can change:
    // print '<pre>' . htmlspecialchars( var_export( $sorted_menu_items, TRUE ) ) . '</pre>';

    $post    = $latest[0];
    $content = empty ( $post->post_excerpt ) ? $post->post_content : $post->post_excerpt;
    $link    = array (
        'title'            => $post->post_title,
        'menu_item_parent' => 0,
        'ID'               => '',
        'db_id'            => '',
        'url'              => get_permalink( $post->ID ),
        'classes'          => array (
                0 => '',
                1 => 'menu-item',
                2 => 'menu-item-type-post_type',
                3 => 'menu-item-object-post',
                4 => 'latest-post',
            ),
        // strips all tags and reduces the length to 20 words
        'attr_title'       => wp_trim_words( $content, 20 ),
    );

    $sorted_menu_items[] = (object) $link;

    return $sorted_menu_items;
}

If we write a new post …

… and call the nav menu like this …

wp_nav_menu( 
    array( 
        'theme_location'  => 'primary', 
        'add_latest_post' => TRUE 
    ) 
);

… we get …

The nav menu item has a class latest-post, so we can style it per CSS:

.menu .latest-post a
{
    color: #eee;
    background: #9f0;
}