Drupal - Change templates based on path alias

You can add some logic in a hook_preprocess_page() implementation that checks the alias, and adds theme hook suggestions if the path matches, e.g.

function MYTHEME_preprocess_page(&$vars) {
  $alias_parts = explode('/', drupal_get_path_alias());

  if (count($alias_parts) && $alias_parts[0] == 'mydirectory') {
    $vars['theme_hook_suggestions'][] = 'page__mycustomtemplate';
  }
}

Then if you create a page--mycustomtemplate.tpl.php file in your theme, and clear the cache, any pages with an alias beginning mydirectory/ will use that new file instead of the standard page.tpl.php.

It's actually easier than that using drupal_match_path() which is how the blocks module works out whether a block should be displayed on a page or not:

function MYTHEME_preprocess_page(&$vars) {
  if (drupal_match_path(drupal_get_path_alias(), 'mydirectory/*')) {
    $vars['theme_hook_suggestions'][] = 'page__mycustomtemplate';
  }
}