In globalredirect.module for drupal 6, I believe that drupal_get_path_alias needs to be added.

I changed
if (!empty($term) && ($term_path = taxonomy_term_path($term)) != $request) {
to
if (!empty($term) && ($term_path = drupal_get_path_alias(taxonomy_term_path($term))) != $request) {

Otherwise example.com/custom/term/path redirects to example.com/taxonomy/term/123, where as the opposite is the intended behavior.