Phr,
Assuming that there were a large NSS position (let's say 500M or 1B, whatever the number may be), and they somehow tried to cover that during normal trading hours. I think by trying to buy any kind of large quantity that would cause the price to spike, at which point they have to walk the price down again.
If they do 500-1000 share transactions to walk the price down as a NSS, they might be able to achieve that. But how are they going to get the large amount of shares needed to cover? We know that large longs are not selling shares, so there wouldn't be anywhere to get them, the few shares that traders unload would be a low number.
So, in summary, I think the buying pressure they create by trying to cover shares would overwhelm any downward pressure they could create when trying to walk it down again, so that most likely the large NSS shares have not been covered yet, if there were some there to begin with, IMO.