Im no expert but i think they sued custom models/skins to 'build' the UI with dodads and unit, Vuen's DnD did something like it, there's a map floating around here someware i think, and my maze maker map has it (way far frmo finished). Hope this helps!
PS: I can get you the map by e-mail stripped of msot triggers so as to give you a 'tutorial' on it, if you want.
Edit: JASS is not realy needed, but that;s if you not using some of the awsome UI's i've seen, just an 'ok' one that does the job