This might work:
/* getUrlParam snippet */
$param = $modx->getOption('promo', $_GET, 0, true);
return $param? 'special-tpl' : 'standard-tpl';
You may run into trouble with the results being cached. If that’s the case, try calling both the snippet and pdoResources uncached.
An alternative would be to call a snippet higher up on the page that set a placeholder based on the URL param and use a placeholder tag for that placeholder as the value of the
&tpl property. That might avoid any caching issues.
In any case, the same Tpl chunk will be used for all resources aggregated by pdoResources. I assume that’s what you want.