TC, people were trading/duping the preorder heads/skins long before the preorder stuff was up for sale on the marketplace. It was the only way to get them--and is a side-effect of them being in the game code the way they are.
Concerning the new heads/skins, I don't think they'll be dupable/droppable, at least on xbox. I've used save state editing in the past and to my knowledge there's no account of the heads/skins being in the game via Gibbed. Even if you could make a droppable unlock for the DLC heads/skins, you should still need to download the files to show them, which is probably not going to happen unless you just bought them to begin with (or had a hacked/modded BL2, which is much easier said than done on xbox).