Is it possible to *optionally* override a theme in Drupal 6?

Answer 1

You could use sections module, or look at how it switches theme for certain menu-items.

Answer 2

I've had mixed success doing template.php menu overrides to force CSS classes and ids or HTML into the output.

You could make use of Block Theme when enabling the menu as a block, but I've never tried it. http://drupal.org/project/blocktheme

If you want to tackle the template way, here are the zen menu override funcitons...

function zen_menu_item_link($link) {
  if (empty($link['localized_options'])) {
    $link['localized_options'] = array();
  }

  // If an item is a LOCAL TASK, render it as a tab
  if ($link['type'] & MENU_IS_LOCAL_TASK) {
    $link['title'] = '<span class="tab">' . check_plain($link['title']) . '</span>';
    $link['localized_options']['html'] = TRUE;
  }

  return l($link['title'], $link['href'], $link['localized_options']);
}

function zen_menu_local_tasks() {
  $output = '';

  if ($primary = menu_primary_local_tasks()) {
    $output .= '<ul class="tabs primary clear-block">' . $primary . '</ul>';
  }
  if ($secondary = menu_secondary_local_tasks()) {
    $output .= '<ul class="tabs secondary clear-block">' . $secondary . '</ul>';
  }

  return $output;
}