umbrella list: unsubscribe key public

Mailman offers a feature to set up an "umbrella-list",
which is used if a list just has other lists as members.

Assume we have an umbrella-list U, which has to
members, X and Y - both are also lists.

Now, cleverly, the monthly password reminders are
not sent to X and Y, but to "X-owner" and "Y-owner"
instead, so this information should reach the owners of
the subscribed lists (sending a password reminder to
X would mean giving each member of the X list the
power to unsubscribe X from U etc.).

But: If a member of X or Y opens the member options
page (http://my.server.net/mailman/options/U/X) and
clicks on the "unsubscribe" button, the necessary key
is mailed to X (and not to "X-owner"). This is probably
not what we want. If the list archive is not private, the
password is even available to everyone out there.