Yet another 'Just called Xbox Support' thread. This time with actual information on what you can do to get your code.

The representatives have definitely been given some information about it and have been told to add certain information to a list. Once she verified my personal information, she asked me what level I was in-game (dafuq?). I said 70. She confirmed my gamertag with me once again, and then went to confirm the email that I wanted it sent to. Here's the thing, she read me my old, old, old email that the account was originally created with six years, even though both my Live ID AND my contact email are different. I corrected the email for her and she added that information to the "list". She said that it's being looked into and when they have any more information for us that the people on the list would receive an email.

tl;dr: If you haven't received a code, and you should have, call Xbox Support and explain that you should have received an email containing a code for downloadable content for Halo 4, but you did not receive the email. If they try to give you shit about not knowing anything about it or it being 343's responsibility, politely ask them to check their procedures/documentation about the issue and that you want to be added to the list that is being compiled.

tl;dr: Call Xbox Support, ask to be added to 'the list'. Make sure you confirm that the email they are adding to the list is one you have access to