Guys are overthinking this.
If you download a game from AUS/NZ store DLC from US/UK store will still work with your game.
The license you get when you buy on the Xbox One store goes for all stores in all regions the content is available.
IF you got Destiny from the US store, but want to download the AUS/NZ version for the early unlock you can:
Just purchase the US version, then change region to AUS/NZ, manage destiny, go to Install, you will now download the AUS/NZ version of the game which unlocks early.
As for which server it will send you to, it will connect you to the closest server so if your console is physically in the US even if the region is set to AUS/NZ you will still connect to an american server.
Matchmaking is IP based, only the marketplace doesnt care about IP.
P.S Ive tested the downloading from other markets and playing online with my Xbone already, it works and no issues so far.
Im guessing once the Xbone is released all around maybe they will lock the marketplace to accounts again.
You can also use the HK store for even bigger saving fyi