Xbox Originals are region locked, d'oh!
While we were busy marveling over all the fabulous additions in the Fall Update, others out there were eager to check out said features in new and exciting ways. First and foremost, the folks at Siliconera were keen to try out the Xbox Originals available to Japan. Knowing full well that gamers with Japanese accounts can download Japanese demos such as the Eternal Sonata demo released in May, they figured a similar method would work for Xbox Originals. With this in mind, they set out to download Magatama, only to discover an error message claiming the game wasn't available in their region (i.e. the U.S.). After a quick console restart, the option to download the game had disappeared, though the free content like themes and pics was still available.
So there you have it kids, your dreams of downloading wacky Japanese Xbox games have been crushed (unless you live in Japan, of course).