HoW looks identical to BG2 in terms of the screen res, character models, sprites etc. You put "Dead Man's Face" on and it now looks like the "Helm of Balduran" on screen. They picked and chose what they wanted from BG2 and added some stuff of their own. BTW, hasn't ToB got scroll cases etc from HoW (and didn't HoW just steal that idea from the Bag of Holding?).
I think the spells are a different matter. They specifically wrote new spells for HoW - quite different to those in BG2.
So if they really wanted to, I think they could do the same with classes and kits in a very short time frame. Seems like a logical conclusion to me.
Cheers Aegis.