Sample code - add menu item with pre-added classes (possibly useful if paired with custom walker that checks for a class and changes output accordingly)

gistfile1.php

<?php
/*
 * Plugin Name: Linkless Menu Items
 * Plugin URI:
 * Description:
 * Version:
 * Author: Kailey Lampert
 * Author URI:
 * License: GPLv2 or later
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 * TextDomain:
 * DomainPath:
 * Network:
 */

$linkless_menu_items = new Linkless_Menu_Items();

class Linkless_Menu_Items {

	function __construct() {
		add_action( 'admin_init', array( &$this, 'admin_init' ) );
		add_action( 'admin_footer-nav-menus.php', array( &$this, 'admin_footer' ) );
	}

	function admin_init() {
		add_meta_box( 'linkless_links', __( 'Linkless Links', 'linkless-links' ), array( &$this, 'add_meta_box' ), 'nav-menus', 'side', 'low' );
	}

	function add_meta_box() {

			echo "<p><input type='text' name='linkless-link' class='linkless-link widefat' value='' /></p>";

			?><p class="button-controls">
				<span class="add-to-menu">
					<input type="submit" class="button-secondary submit-add-to-menu right" value="<?php esc_attr_e('Add to Menu', 'linkless-links' ); ?>" id="submit-linkless-link" />
					<span class="spinner"></span>
				</span>
			</p><?php
	}

	function admin_footer() {
		?><script>
		jQuery('#submit-linkless-link').click( function(ev) {
			ev.preventDefault();
			linklesslink = jQuery('.linkless-link');
			label = linklesslink.val();
			console.log( label );
			wpNavMenu.addItemToMenu({
				'-1': {
					'menu-item-type': 'custom',
					'menu-item-url': '#',
					'menu-item-title': label,
					'menu-item-classes': 'no-link' // custom classes added here
				}
			}, wpNavMenu.addMenuItemToBottom );

		});
		</script><?php
	}

}