How can I get recaptcha to protect the blog module comments?

I have installed the spamprotection & recaptcha modules, and I have successfully got them running on some customised forms (a contact us form).
http://www.silverstripe.org/all-other-modules/show/267893#post267893

I cannot however, see how to get the recaptcha to appear on the blog module comments.
I have had to add MathSpamProtection in the meantime, but I would prefer the recaptcha

Here's the /mysite/_config I am using :

SpamProtectorManager::set_spam_protector("RecaptchaProtector");
RecaptchaField::$public_api_key = 'code';
RecaptchaField::$private_api_key = 'code';
 
// field creation (Should this even be here?)
$recaptchaField = new RecaptchaField('MyCaptcha');
$recaptchaField->jsOptions = array('theme' => 'clean'); // optional

PageComment::enableModeration();
MathSpamProtection::setEnabled(); 

any suggestions?

this is an old post but heres the solution for ss 3.1. it'd be easy to turn this into a module too.

:::php
class CommentSpamProtectionExtension extends DataExtension {

	public function alterCommentForm(Form $form){
		SpamProtectorManager::update_form($form, 'Comment');
		return $form;
	}
}

::yml _config.yml
CommentingController:
  extensions:
    - CommentSpamProtectionExtension