You might be able to try a str_replace for links in messages:
PHP Code:
$find = '<a href="http://';
$replace = '<a target="_blank" href="http://';
$this->post['message'] = str_replace($find, $replace, $this->post['message']);
$find = '<a target="_blank" href="http://www.yoursite.com';
$replace = '<a target="_top" href="http://www.yoursite.com';
$this->post['message'] = str_replace($find, $replace, $this->post['message']);
I don't like it, but maybe it will give you an idea to go about it another way.