Make drupal_html_to_text themable

The details of how HTML is converted into plain text is a question of personal preference. Why do I need to have all h1 and h2 tags converted to upper case? What if I need other line lenghtes? There should be a way to get to these styling issues, at least via a hook_html_to_text, although some sort of template_preprocess_mail could make things much easier.