I just found an detailed howto
(you have to scroll down a little bit) for setup mailman with postfix.
Maybe it can be used to build an mailman package for ISPConfig. I'm not sure if its possible that each client could have their own lists (== for the domains the client controls)
I also saw a table mail_mailman_domain in the ISPConfig database, which isn't used yet, right?
I'll try to get it to work, but its not on the top of my todo list