It seems that there are no product tokens available for use in the threshold mail. I.e. I tried [model] which doesn't get replaced. I think it's because of line 245 at uc_stock.module:
$token_filters = array('global' => NULL, 'order' => $order, 'stock' => $stock);
Function _uc_stock_send_mail() is documented with three parameters "$order, $product and $stock". But it's implemented with $product missing.
Please add this parameter, thanks!