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!
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.
People are claiming doing this on xbox actually corrupts their save—so beware, and maybe copy your save first if you're trying it there.
See this GameFAQs post for details, or a video of the heart disintegrating on Youtube.
The questions still remains...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?