also presumably it would be pretty obvious to everybody involved that this was happening, as it would take so much collaboration
Actually, it would be obvious not because of the collaboration required, but because all the pools that are
not in on it will notice their blocks are being ignored. It will be noticed pretty much instantly, and once the miners of the attacking pool realise their pool is performing a 51% attack, they'll leave
en masse for other pools, as almost all miners are either in it for the money or for the benefit of the network, and neither group wants any part of 51% attack (the first group doesn't want the massive price drop that a 51% attack is likely to cause, and the second group doesn't want a 51% attack simply as a matter of principle). When they leave, the attacking pool no longer has 51% and the attack is over, hopefully before they were able to cause any real damage.