Meh, Genocide, you're taking the difficulty with the Infliction ability way to far.
Yes, I understand you like the Woren idea better then the GD idea, you happen to be the main enthuasiast for the idea, but really, it's not as big a deal as you seem to be implying.
Now, the reason there is no 'such amulet' in GD is because every mage and their mother can ward, even the Werewolves living on Earth can ward. It's seriously not a hard thing to have done.
Now in the Narutoverse it would be a little bit harder, no doubt, but I seriously doubt it would be as hard as you seem to implicate. Sure, the Death God sealing technique isn't it, and I'd laugh if some one said so, but just suggesting it seems like an attempt to put to detail a warding technique that just doesn't need it.
Now, I can see the Yondaime performing the warding just as soon as Naruto comes out, but given the time constraints with Kyuubi and the like, I'll grant it's feasible he wouldn't have the time. However, I sincerely doubt Sarutobi didn't know Arashi was a were-cheetah, or about the *basics* of his kind, and make no mistake, the ability to inflict is the most *feared* aspect of most Were-creatures, everyone knows who has even a cursory knowledge of were-creatures knows about it.
For Simplicity sake Sarutobi could seal the ability to inflict. Simple enough explanation that doesn't require a convulting story point. I really fail to see the problem with using GD style Were-creatures (Cause thats all I'm seeing is wanted for this particular cross).
Meh. I'm more a fan of GD then I am of Woren, but I'm not unhappy with either series. I don't see a problem with using either, given a little explination.
EDIT:
And as for the complete lack of magic, understand the GD is a realm of different dimensions. There have been people dimension hopping to and from GD, it's easy to come up with a mis-happed dimension jump could easily explain the presence of some were-cheetahs in the Naruto-verse