Authcache: Getting user_link to work in template.php
Ok so here is my solution.

Menu's are themed in MYTHEME_preprocess_page, the menu data is also still structured here unlike in MYTHEME_preprocess_menu_tree.

I enabled the memcached example module, this enables the variables user_link and user_name in the preprocess. Then I created a third variable user_account just copying from the other two and altered the cookie so it stores uid not name.

This is because I wanted the "My Account" link to link to "user/x/edit" and I didn't need the name.

I then added the following chunk to my page preprocess.

function MYTHEME_preprocess_page (&amp;$vars) {
  $user_account = $vars['user_account'];
    if (array_key_exists('menu-xxxx', $vars['secondary_menu'])) { 
      $vars['secondary_menu']['menu-xxxx']['href'] = $user_account; 
    }


The secondary menu in this case is my user menu and My Account is always labelled menu-xxxx in the array. (xxxx will be numbers).