I do this in Boost 7.x. It's not pretty, but it does get the job done.

[hook_page_delivery_callback_alter][1]

[boost_page_delivery_callback_alter][2]

[boost_deliver_html_page()][3]

boost_deliver_html_page() is on line 1181 of the above link (#l1181). I can't post more then 2 links due to spam prevention.


  [1]: http://api.drupal.org/api/drupal/modules--system--system.api.php/function/hook_page_delivery_callback_alter/7
  [2]: http://drupalcode.org/project/boost.git/blob/refs/heads/7.x-1.x:/boost.module#l263
  [3]: http://drupalcode.org/project/boost.git/blob/refs/heads/7.x-1.x:/boost.module#l1194