Within the published book Halo 2 Hacks, as part of the envy skull hack #13 (Camoflauge the Master Chief), it states that you can use the Get on top of Delta Halo hack (hack 33) to get to the Envy skull. This is not possible because if you go on top of the level and walk to the temple area, you will realize that NONE of the objects are there, including the skull and the two invisible elites facing each other. This is because the objects/enemies were not loaded at this point in the game. You MUST go through the entire level (without using the Get on top of Delta Halo Hack) on legendary in order to load the skull.
See also:
http://www.oreilly.com/catalog/halo2hks/index.html
this is accurate because you can skip the first section of the map and still get the Envy skull. if you try to skip everything before the temple, you will not trigger the spawn point for the skull. but you can go around the first building and thereby skip the entire first fight - allowing you to bypass numerous elites and snipers. just make sure that you go down the cliff before the bridge (where you get the scorpion).
complete errata for the book: http://geeklit.blogspot.com/2005/11/halo-2-hacks-errata.html