At the moment it's not possible to get $_info data from a Zend_Db_Table_Select instance. A possible solution could be a getInfo() method, which is a slightly modified copy of the getPart() method:

/**
 * Get part of the structured information for the currect query.
 *
 * @param string $part
 * @return mixed
 * @throws Zend_Db_Select_Exception
 */
public function getInfo($info = null)
{
    if ($info === null) {
        return $this->_info;
    }

    $info = strtolower($info);
    if (!array_key_exists($info, $this->_info)) {
        require_once 'Zend/Db/Select/Exception.php';
        throw new Zend_Db_Select_Exception("Invalid Select part '$info'");
    }

    return $this->_info[$info];
}

The idea of fusion getInfo() and getPart() together crossed my mind as well, but the data in $_info is too different. However, there is some info in $_info that does/could belong to $_parts, such as the source table name. Something like getTableName() method for both Db_Select and Db_Table_Select would be nice in this case, because this would provide a generic interface to this data.