Sweet, though I imagine that'll probably get patched soon.
hopefuly someone else makes a thread but basically :
1. make a dummy account, set the region to japan (this is only if youre scared your main account may get banned)
2. buy xbox credit (yen) from microsoftstore (set region to japan, search for yen cards on the site. they will be sent to you within seconds)
3. buy the game, enter a japanese adress, i used the us embassy in tokyo
4. set your xbox region to us or uk or wherever you live
5. download the game