How hard is it to create a stub forwarder dll for setupapi and cfgmgr32?
Not sure what you mean exactly. cfgmgr32 should forward to setupapi already, and I sent a patch that does that for the few stub functions that it had. setupapi's CM_* stubs could be improved, but I don't think you'll be able to forward them to any native code to try out: I expect the Windows configuration manager is rather too low level to run under Wine.
You'll probably want to create a hacky version of setupapi that creates a fake device tree, with a root, USB hub, and iPod device, just to see if that makes iTunes happy.
Does that answer your question? --Juan