Bill,
IMO there is a current in the water pipe almost all the time anyway. The pipe between the old and the new house will be in intimate contact with earth, and, depending upon soil conditions, the earth will be a parallel return path to the xformer. If the neutral to the "new" house goes open and all the return current is forced into the water pipe and supplemental ground (presumably a ground rod), the "new" house will start showing voltage swings. The voltage swings in the connected load in the "new" house will be their greatest as the impedance to ground through the water pipe and supplemental reach their maximum. Kinda' creates a flashing alarm circuit, IMO. A great many of my customers are already worried about their systems when they can see the lights flutter when the refir or washer motor starts.
The plumber, poor guy, is given short shrift by the NEC. Case in point.
There is an island in the Mississippi that has had housing on it since the late 1800s. The soil of it is just deep enough to dig a basement before hitting limestone. The limestone is about 12 feet thick and sits on top of a deeper layer of sandstone, all of this above the normal water level of the river. When water and sewer were added to the island (about 40 now historic buildings), it was done by digging a brick lined tunnel through the sandstone big enough for a man, hunched over, to walk through. Water and sewer lines were then installed side by side on the floor, the man now forced to straddle the lines as he walks. In order to cut in or replace a water service, procedure requires the use of clamped on cable jumpers, and woe to the guy who doesn't.
Al
