Codeigniter style validation callback function not working

I've created an EE custom addon that processes a simple enquiry form. All works well but I'm trying to add a callback function to my validation rules and it appears to be completely ignored.

Here's my call to the validation rule:

ee()->form_validation->set_rules('url', 'URL', 'callback_honeypot'); // Honeypot field

and my callback function (which is in the same file) is:

public function honeypot($input)
{
    if (!empty($input))
    {
        ee()->form_validation->set_message('honeypot', '<p>Not all is as it should be...</p>');
        return FALSE;
    } else {
        return TRUE;
    }
}

Will the standard CI form validation callback method not work with EE?

Update / workaround:

Although I haven't got the callback method to work, a simple workaround for (in my case) a honeypot field is to hide the field on the frontend form and use something like this:

ee()->form_validation->set_rules('nickname', 'Nickname', 'max_length[1]');

So if the 'nickname' field has more than one character entered into it, the validation fails.

Alternatively, you could do this:

$honeypot_field = ee()->input->post('nickname', TRUE);

and only process the form if the following conditions are met:

if (ee()->form_validation->run() == TRUE && (empty($honeypot_field)))

But it doesn't answer the question of why the proper CI callback functionality doesn't work in EE.