I think you should be able to use newpost_process and check $post['title'] and $post['message']. If you detect an error you can add an error phrase name to the error list like this:
PHP Code:
if ( /* put your check here */ ) // something wrong with title or message text
{
$dataman->error('error_phrase_name');
}
Of course you should chnage 'error_phrase_name' to the actual name of your phrase. The error phrase should be in group "Error Messages".