Well, last time the talking here was that due to QT license, KHTML couldn't be used, and now with the Safari you don't need any QT (you can use other GUI kits around)...
Actually Ender was de-Qting KHTML at some point, are you still around Ender? I haven't seen the code but I expect you can forget about Safari - iirc it's been integrated with Quartz which is of course a proprietary Apple system only. Seeing as they were ranting about speed, I doubt they'll have imposed the overhead of a graphics layer abstraction system.
About duplication - I think it's probably most likely that the best course would be to copy the KHTML source into the Wine tree, because I'd expect modifications would be needed in order to support all the IEisms that will crop up in regular usage of the WebBrowser control. KHTML out of the box won't, and never will support ActiveX, IE specific markup or any of the other proprietary stuff MS threw in for fun during the browser wars. The KDE team would rightly reject such patches from being integrated into upstream.
I don't know how much progress Ender made, it might be worth emailing him to find out.
thanks -mike