foo.php

<?php

$dataList = $dataList->alterDataQuery(function($dataQuery) {
    $selectSQL = '';
    $alias = '';

    // Hack - DataQuery::selectField() is protected in SilverStripe 3
    // $dataQuery->selectField($selectSQL, $alias);
    $reflectionMethod = new ReflectionMethod($dataQuery, 'selectField');
    $reflectionMethod->setAccessible(true);
    $reflectionMethod->invoke($dataQuery, $selectSQL, $alias);
});