I am either a Slowpoke or I didn't search well enough but I haven't seen any thread about this. But basically my friend today was playing Pokemon Alpha Sapphire and he got a shiny Beldum from the mystery gift. So with a quick search I found out this is an official promotion happening from November 21st to January 14, 2015.
All you need to do is:
Select Mystery Gift on the Main Menu
Receive Gift
Get via Internet
Go to the Pokemon Centre and talk to the delivery girl
Then a Shiny Beldum holding a Mega Stone is all yours!
http://www.pokemonrubysapphire.com/en-ca/pokemon/get-a-shiny-beldum