I think I may have found a bug in the Director.php code. Correct me if I'm wrong.

If I use Director::addRules() to specify a custom controller for a URL like this:

Director::addRules(50, array ('myurl/$Action/$Class' => 'My_Controller'));

The getControllerForURL() function in the Director class will parse that array and get the name of the controller as a string ("$controller"), and it will instantiate the "My_Controller" object like this:

$controllerObj = new $controller();

However, every descendant of the Controller class expects a "$dataRecord" variable to be passed into it. Silverstripe errors out when I use anything but "Controller " as my custom controller because the __construct() method is missing argument 1.

Am I missing something? Seems crazy!

Hmm.. Never mind. It seems the only accepted custom controller you can use is a subclass of Controller.