Moving to Mailman-Developers:

...

It should go without saying that it's very interesting; this is a FAQ [...]

Woo-hoo. That's encouraging. Well, if the only thing, that prevented the feature from appearing by now, is lack of development resources, then I'll get right on it. Thanks,

Well, it's not a lack of development resources alone. It's one of those "in principle" things, although not a religious principle (eg, you won't get the "this is evil" response from anybody the way you would with advocating Reply-To munging).

The problem, in brief, is that the design of Mailman 1 and Mailman 2 is distribution-centric. They manage rosters of subscribers on behalf of a list. What you say you want is a program that manages groups of lists on behalf of a user. But this isn't quite good enough. What you really want is ... Usenet news, except on a push basis.

Doing this efficiently and maintainably is going to require a global roster of users, which is something that Mailman 3 will provide. There are some simple, not-too-unclean hacks that can be done, but I think Mark Sapiro's "sister lists" feature is about the best that can be done.