Hey Josh.
I was able to make a small modification to your instructions and I got this mod to work with 6.4.2.
All I did was remove the following line from the modified code for addpost.php:
$header = $mailer -> headers();
Then I replaced the old call for sending mail:
mail("$to","$subject",$msg,$header);
with the new call:
$mailer -> send_mail($emailformat,"",$to,$subject,$msg);
and lo and behold everything worked! So now my users (who want it) are getting instant email notification on all posts in the Forums they are interested in.
Thanks a ton for posting this original mod. I'm loving 6.4.2 and now my users are too.

Take care,
Geoff