So I decompiled FEZ.exe and found codes that only work when you're standing in the heart room beyond the 64 cube door...
You actually enter the codes in the menu screen—there's a separate code for both the letters and numbers artifacts. When you do both, the heart disintegrates, and the game does a faux-reboot!
The code to disintegrate the heart is (and BE WARNED: this worked fine on my Steam copy, but people are saying doing this on xbox actually corrupts their save—so beware, and maybe copy your save first if you're trying it there):
- Get all the heart pieces, and go to the "Temple of Love" behind the 64 cube door.
- Open the menu, go to the artifacts page.
- Look at the red "letters" artifact and enter
LT RT RT LT RT LT LT LT.
- Look at the green "numbers" artifact and enter
RT RT RT LT RT RT LT LT.
But I can't find anything else, past that. Nothing seems to change, and there's no extra ending. The heart room does remain empty though.
See this GameFAQs post for details, or a video of the heart disintegrating on Youtube.
Questions still remain...how were we supposed to figure these codes out? Were they "cut" from the game? Do the faces of the artifacts lead to them? What is the purpose of the alien skull artifact?