You can't populate native *displaceable* objects of any kind (including rocks). That's the limitation. It's related to the overall issue with ray tracing of displacement, I believe.
You can populate the built-in Rock object because it's one of the populatable types, not the fully displaceable type. But if you're building rocks from built-in spheres, they can't be populated yet.
That is why I initially questioned this Matt. I did not understand why you could assign # of faces and subdivide and shatter the built in rock object and populate it, but not somehow attach APs displacements. I will accept that it cannot be done after all these great answers, but I was curious. Sounds like we shall have more wonderful stuff soon.