Extra params for custom error messages

To allow for things like this:

//...

$form['myfield'] = array(
  '#type' => 'textfield',
  '#title' => 'My Field',
  '#required' => TRUE,
  '#rules' => array(
    'length[10, 50]', 
    array(
      'rule' => 'alpha_numeric',
      'error' => 'Please, use only alpha numeric characters at %field with extra an extra %param.'),
      'error params' => array(
        '%param' => t('Some param'),
      ),
  ),
);

//...

This instead of passing dummy data in the field and adding an error callback to generate the error message manually. I think this is a relative easy addition to fapi_validation_rules_execute() with some nice extras :)