Hi everyone, thank you for answering my question.
My understanding from your replies is that I will not be able to get Darkrai without a Japanese 3DS and game, and that I won't be able to get Diance unless a) the event is not region-locked, or b) I purchase a Japanese 3DS and a Japanese cart. As such, the best course of action is probably to wait until the promotion starts, and then check back here once someone else in Japan reports on the regional status of the event.
Although it is not my fault, I am sorry that my inquiry seems to have sparked a bit of an argument regarding "legitimacy" and "legality".