"Subscribe all" / default subscription feature

Issue Summary

Hi,
is there a possibility to
a) subscribe all registered users by default to a mailing list?
b) set a default subscription for all future users

If not, I would be happy to add this feature.

Is this a valid approach for subscribing current users:
In mailman_manager_admin_form(): Add functionality for "Subscribe all users to this list"

In mailman_manager_admin_form_submit(): Loop over all users and call _mailman_manager_update_subscriptions() for each uid.

And for subscribing future users, something along the lines of the default subsciption in the simplenews Newsletter module, like described here: http://drupal.org/node/76648

Regards,
Jutta