I'll just jump in and confirm what Cairenn posted earlier.
Currently (emphasis on "currently") Turbine only allows skinning (image replacement) of the UI.
Full on scripting of UI's ala Auctioneer, Outfitter, or any other WoW mod involves having the game engine and the UI plugin communicate. The Turbine engine is not currently set up to do that - you can't just flip a switch and "hey, look at me scripting UIs!"
It's a significant development task to enable. However, with that being said, it's something that we all want to devote some time to doing. The "skinning" ability was something we needed to see if was even viable as a first step to full scriptability.