Filter Content on all Post Types

All I can think is to process the posts at an earlier stage with a Core hook. This is crude, of course, but:

function hack_the_post($posts) {
  foreach ($posts as $p) {
    $p->post_content .= ' I added this';
  }
  return $posts;
}
add_action('the_posts', 'hack_the_post');

It is a bit energy intensive but so long as your plugin/theme uses more or less standard core functions it should work.