How to make a menu link on a subdomain use the main domain as base_url
It will depend on your theme, but typically the menu array (e.g. $mail_menu in page.tpl.php) can be altered such that each menu item have the array element 'absolute' set to TRUE so that l() will generate the link with an absolute path.

For example, in a MYTHEME_preprocess_page() function:

/**
 * Override or variables into the page template.
 */
function MYTHEME_process_page(&amp;$vars) {
  foreach ($vars['main_menu'] as $id =&gt; $menu_item) {
    $vars['main_menu'][$id]['absolute'] = TRUE;
  }
}


As long as $base_url is set to http://example.com, it should render the menu links to your original domain.