[kh99]

Well, when you're submitting a new post, it goes to newpost.php with do=postthread, so maybe you could use hook newthread_start with code like:

if ($_REQUEST['do'] == 'postthread' && /* check for validation not done */)
{
    show validation page (i.e., render a template and call print_output(), or do a redirect)
}

I hope that makes sense.