It probably will never happen but... Imagine the GB recieved some new light that told them the great tribulation was imminent. The world is going to hell and all JWs can expect until armageddon is terrible persecution the likes of which no Christian has ever experienced in two thousand years. But Jehovah has a solution: put yourself and your family 'to sleep' and after armageddon's over, Jehovah will 'wake his people up' at the resurrection. How many JWs would be willing to go through with it. None? A few? Most? All?
Considering how many JWs have allowed their own children and spouses to die when they needed a blood tranfusion, I'm betting most would happily chug the kool-aid and die with a big, bovine grin on their faces.
Frankly, this is one of my nightmares. I know it seems improbable now, but keep in mind that even Heaven's Gate explicitely taught that suicide was wrong.. until Applewhite got his new orders from the saucermen. It wouldn't be hard to pull off either. The Org has a very efficient private grapevine that allows them to swiftly and discretely communicate with any congregation in the world. No need to publish anything in the official publications. It could be done in a matter of hours.