Climb & Return

function climb_and_return($elem, $test_fn, $default) {
  
  $test = false;

  while(!$test && $elem !== 0) {

    $test = call_user_func($test_fn, $elem);
    if ($test == false) {
      if (is_page()) {
        $elem = $elem->post_parent == 0 ? 0 : get_post($elem->post_parent);
      } else if (is_single()) {
          if ($elem->post_parent) {
            $elem = get_post($elem->post_parent);
          } else {
            // theoretically we shoudl climb to the taxonomy level now
            $elem = 0;
          }
      } else {
        $elem = 0;
        $test = false;
      }
    }
  }

  if ($test != false) {
    return $test;
  } else {
    return $default;
  }

}

/*----------*/

function my_has_featured_image($elem) {
  if (has_post_thumbnail( $elem->ID )) {
    return get_post_thumbnail_id( $elem->ID );
  } else {
    return false;
  }
}

/*-----------*/

$thumbnail_id = climb_and_return($post, 'my_has_featured_image', 142);