When a superlist is mailed, the sub-lists themselves are not really recipients -- just the merged members of those sub-lists.

To have an archive, you must send to that list (which possibly could be a subscriber being careful not to trigger the anti-spam heuristics).

It may be that a separate, archive-only (no subscribers) llist would be required and then those archives would be accessible (with appropriate access controls) to the subscribers of the sub-list.

/Pete