discarding incoming messages with no ack

I'm trying to gateway two different mailing lists to each other (stupid,
I know, but it's a historical thing, and forwarding one to the other and
combining the subscriber lists wasn't an option for some reason).

Of course, the easiest way to do this is just subscribe both lists'
posting addresses to each other.  Majordomo has mighty fine duplicate
detection so this *almost* works out well...

The problem is that Majordomo also sends an ack to the sender telling
them that their message was flagged as a duplicate (and throws it in the
moderator queue by default).

I've managed to get it to not go into the moderator queue by adding the
following to access_rules:

post
deny
$dup_msg_id

However, this still sends an ack to the sender telling them their post
was rejected.  I know you can globally disable the acks, but I don't
want to do that, because I still want senders notified if they get
blocked for some other reason, I only want it disabled in this one case.
 I've been digging through the help files for access_rules and
access_variables for an hour now trying to find something that would
pull this off (replyfile with a nonexistant file, forward to nobody,
etc), but it still winds up sending an ack to the sender.

Is there something that's not in the documentation that would let me
disable the notification? (or is it there and I'm blind?)

Thanks!
-- 
Dave