Once you have a skinning API, you just implement a 'gnome compat' and 'kde compat' skin (or even just an automagical one that does both) the thing would "just work", right? Why, it shouldn't take more than a couple of hours ;) (WARNING: that last part was a joke, if you set your watch by this you very well may miss work for several years).
I don't think we could do KDE skins, as (surprise) they depend on Qt which is GPLd.
Well, I can't see why Trolltech (the owners of Qt) would object. Presumably the main reason that you would want a Qt skin for Windows application running under Wine is that you run several other Unix applications using Qt... So I can't see why they wouldn't give us special permission if we for some reason would need it... The alternative a GTK skinned only Wine would not benefit them at all, quite the opposite.