Setting menu trail of subpage of a node
The function you need is menu_tree_set_path.  It will be released with Drupal 7.9.

If you can't wait, the workaround is to use this code in hook_init:

&lt;?php
$item = menu_get_item();
$item['href'] = '&lt;the href of an existing menu link&gt;';
menu_set_item(NULL, $item);
?&gt;