I also wonder if what they say about undead monks would apply to an undead creature that became a magical beast by a template... Would it be Undead (augmented magical beast) or Magical beast (augmented undead)?
If I was completely free to choose I'd make it an Elemental (shadow), because of how shadow is considered an element in my campaign but not sure if that's possible.
I figured it out by looking at similar cases explained in the FAQ (p31).
If a template says its allowed, it probably is (DMs can always rule it isnt), but in the end you look back at every type the creature possessed at one time, and pick the highest. It's just a name though cause the creature will have the traits of all the types it has trough the augmented subtypes.
So I guess this would be Undead (augmented magical beast), if I had not decided not to use the creature at all.