Whilst trying to tidy up the Country select dropdown, as described at http://issues.civicrm.org/jira/browse/CRM-5227 I found myself needing to group the countries using <optgroup> within the select. Though this is possible with HTML_Quickform2, CiviCRM is currently using HTML_Quickform1, and this feature is unavailable. Whilst looking for a solution, I found http://pear.php.net/bugs/bug.php?id=1283&thanks=3 where HTML_Quickform1 has been hacked to add in the support. I rolled up the said changes into a patch (http://pear.php.net/bugs/bug.php?id=1283&edit=12&patch=add-optgroup-support&revision=1255771468) and applied it to CiviCRM (attached). It seems to work absolutely fine :)

For reference, this is how I'm using it, in hook_civicrm_buildForm:

      // rebuild the country selecter to only contain the UK and select UK by default
      foreach(CRM_Core_PseudoConstant::country() as $key => $value) {
        if ($key == '1105' || $key == '1226') {
          $topCountries[$key] = $value;
        }
        else {
          $otherCountries[$key] = $value;
        }
      }
      $finalCountries = array('' => '- select -') + $topCountries + array(' ---------- ' => $otherCountries);
      $form->addElement('select', 'billing_country_id-'. $form->_bltID, ts('Country'), $finalCountries);