Add custom field for menu
You could use the 'not often used' description field already available on each link? Then just add it into the menu links yourself on the front end by overriding the menu_link theme function.

function theme_menu_link(array $variables) {
  $element = $variables['element'];
  $sub_menu = '';
  $description = '&lt;span&gt;' . $element['#localized_options']['#original_link']['description'] . '&lt;/span&gt;';

  if ($element['#below']) {
    $sub_menu = drupal_render($element['#below']);
  }
  $output = l($element['#title'] . $description, $element['#href'], $element['#localized_options']);
  return '&lt;li' . drupal_attributes($element['#attributes']) . '&gt;' . $output . $sub_menu . "&lt;/li&gt;\n";
}