A number of years ago I remember reading an online paper on some Alaskan site about why pipes burst. As I recall it is anytime you confine the water and then it freezes and expands. Simple Physics. The confinement can be simply two sections of pipe freezing and then the water in the middle freezes and the pipe bursts. Such as a water pipe in a crawl space. Your logic sounds right in that with a pipe down to a main drain, even with water up to the pool waterline and therefore above the frost line, the water would start freezing closest to the surface and work its way deeper but the bottom end of the pipe is always open to the pool. Therefore no ice plug and no burst. I guess where one could get into trouble is if there is a horizontal run above the frost depth. Anyone doubt this just stick a vertical piece of PVC in a pool that will freeze and see what happens after the Winter. It shouldn't crack. This thread made me think a little about the main drain issue. More comments welcome so I can include info in the sticky about closing IG pools.
Al