As it sits, the custom item item render hooks have no context so we don't know if the item is being rendered in someones hands, or in the gui, where in the gui etc..
Looking around I came up with a setup that may work. So i'll just leave this here for whenever Eloraam gets free enough to look at it properly.
http://pastebin.ubuntu.com/708334/
Each actual hook would pretty much break down into the last 4 lines of that paste + a texture override. I don't have any sample code for any of these render environments so if anyone wants to donate some i'll give some examples.
Comments?