Dynamic setting of header/footer text in D7 / Views 3

I'm having trouble setting header text dynamically in my custom views using the available hooks. I've seen others with similar issues (e.g. https://drupal.org/node/749452 + comments), but no concrete solution.

I can set fixed headers from hook_views_pre_view() (but NOT hook_views_pre_render()) like so:

    $options = array(
      'id' => 'area',
      'table' => 'views',
      'field' => 'area',
      'empty' => FALSE,
      'content' => $headertext,
      'format' => 'filtered_html',
      'tokenize' => 0,
    );
    $view->display_handler->set_option('header', array('text' => $options));

But what I really need is the same thing from hook_views_pre_render() (or similar) so that I can display stats on the number of results etc, data not available until after the query is executed. I could use the php filter via the GUI to do what I need, but I'm developing a module and don't want to force users allow that.

This worked from hook_views_pre_render():

 $header_options = array(
    'label' => t('Header text'), // Administrative label. Not really needed here.
    'content' => $headertext, // Content of header text.
    'format' => 'filtered_html', // Format of header text.
    'empty' => 1, // Show even when there are no results. Set to zero otherwise.
  );
  $view->add_item('default', 'header', 'views', 'area', $header_options);

... but only for 'default'. Also, if I used default and just changed the text dynamically, other displays get messed up such that, for example, clicking on pager links in the child displays causes a reversion to the default.

It seems like I'm missing something obvious, any pointers?