[MM3-users] Programmatically modify Thread-Topic *before* message is sent out the door?

Thomas Ward via Mailman-users writes:

Programmatically, how would we alter the Thread-Topic during the handler's processing to prepend the corresponding prefix/tag so that when the message actually goes out the door it is properly sending with the modified Thread-Topic?

Same way as you munge the Subject:

msg['thread-topic'] = munge(msg['thread-topic'])