Is there are way to use Director::addRules() to route to a particular action on a controller? For example, I want to do something like

Director::addRules(10, array(
'confirm//$ID' => array('Controller'=>'AccountPage_Controller', 'Action' => 'confirm')
));

So if someone visits /confirm/23 it'll go to AccountPage_Controller::confirm() (and pass ID = 23 in $request).

You don't need to specify an action as anything coming in to the rule will go to the controller::index function.

Director::addRules(10, array(
   'confirm//$ID' => 'AccountPage_Controller'
)); 

This will route anything from /confirm* to the AccountPage_Controller. From there you can process either by action (/confirm/me to AccountPage_Controller function me()) or by handling it in the index() function.

All values passed on the URL will be available in the array $this->urlParams. So with the above rule in place in your AccountPage_Controller:

public function index() {
    if( isset($this->urlParams['ID']) and is_numeric($this->urlParams['ID'])) ) {
          // call some confirmation code with $this->urlParams['ID'])
    }