Since the avowed purpose of the restriction is to protect the privacy of member's content, allowing the archives to be opened to non-members as long as the owner is the only member makes a lot of sense.

I'd go further and allow the archives to be opened to non-members as long as the only postings in the archives were made by owners or moderators; or are older than the date/time the archives were last made members-only. The first condition would allow having a larger team work on the start-up of the group. The second condition would allow an established group to correct an accidental (or malicious) switch to members-only archives - perhaps at the cost of deleting a few messages that arrived after the switch.