This very simple module contains only one externally useful function:
echo_themed_page($title, $content, $theme)
Returns a page themed for the anonymous user.
Generates the text of a fully-themed Drupal page. The rendered page is returned as a text string instead of being sent to the browser. The theme system can thus be used to style any HTML message as if it had been generated by the live website.
Still looking for a way to gracefully avoid the internal drupal_http_request() call but after spending six hours on it, I'm almost convinced that it's not possible.