Presumably you'd fuck it up quite badly. It's an epicyclic gearbox, and those tend not to like the output shaft driving the works very much. Although saying that, there's plenty of mesh gear boxes that can't take coasting in neutral, because the oil pump runs slow and causes damage, so maybe there's that too.
In neutral it would stop oil supply to the gearbox, though it would still be turning, hence wearing the gearbox out quickly. Not just that but you'd be sat on the brakes potentially on a hill if you weren't using the engine braking, potentially depleting the air.