Ok, forwarding from OTRS to mailing lists should now work correctly.
The problem was that when OTRS (along with secure.wikimedia.org) was moved from our main mail server to a second box, the mail configuration on that second box wasn't quite right.
It thought that it controlled mail for the entire wikimedia domain, which it doesn't. As a result, mails forwarded to the lists got handled on the wrong server, where they fell into a deep black hole never to be seen again.
I've tweaked the config so it knows it handles only OTRS, and other mails are forwarded properly through the main mail server.
-- brion vibber (brion @ pobox.com)