In my opinion:
I'd start from today because fuck going back to times immemorial.
I assume the duplication of locked threads would be dealt with the same as it is now.
If only staff could post that should avoid people trolling locked threads.
That said, the utility of such an archive is questionable, don't know that it's something that would be crazy useful, but it would be nice every now and then.
Maybe SpokenTruth could be the one who runs it:P