By looking at the Codeception L4 sample app, I was able to see how to bootstrap the autoload to resolve this issue, by adding these lines to project/app/tests/_boostrap.php:
include __DIR__.'/../../vendor/autoload.php';
$app = require_once __DIR__.'/../../bootstrap/start.php';
\Codeception\Util\Autoload::registerSuffix('Page', __DIR__.DIRECTORY_SEPARATOR.'_pages');
Edit: when upgrading from Laravel 4.0 to 4.1, it is also necessary to add an extra line:
$app->boot();