Though I believe that the corner stone can't be viewed by the public from behind the fence that rings the hole, I'm pretty sure that spot can be seen from the Path.
Comparing the shot I took from the Hoboken train and the grab from that great panorama photo, it looks like it wouldn't be hard to see the cornerstone from the train or even the platform in the station.
Path View (taken some time between Jan and March '04)
I just read the Times piece referred to in the Curbed post and they mention there is a view from the Vesey St. pedestrian bridge. If I'm wandering around this weekend I'll see what kind of view I can find from the station. (I'd rather not get on the actual train if I don't have to. Ug.)