Less reliance on mimemail_prepare in mimemail function

I've been working to make the xpertmailer library work with mimemail as an external mailengine. I'd like to have xpertmailer actually construct the message, however, and the current structure of the mimemail function doesn't allow this, it always uses mimemail_prepare.

It would make things easier if mimemail determined the engine first, and then checked for a custom prepare function. For example:

function mimemail($sender, $recipient, $subject, $body, $plaintext=null, $headers=array(), $text=null, $attachments=array()) {

  $engine_prepare = variable_get('mimemail_engine', 'mimemail') .'_prepare';
  if (function_exists($engine_prepare)) {
	$message = $engine_prepare($sender, $recipient, $subject, $body, $plaintext, $headers, $text, $attachments);
  }
  else {
	$message = mimemail_prepare($sender, $recipient, $subject, $body, $plaintext, $headers, $text, $attachments);
  }

  $engine = variable_get('mimemail_engine', 'mimemail') .'_mailengine';
  
  if (!function_exists($engine)) {
    return false;
  }

  return $engine('send', $message);

  return false;
}