The Xamarin tools have supported using a remote Mac to run the simulator. Microsoft has an iOS shim library and Objective-C tool chain for porting apps to Windows, but it's very much in preview.
Ah, I see. Less exciting, though I suppose inevitable.
If only running OS X as a virtual guest on a Windows host was supported. Then you could use that as the remote mac that is used with the shim.
It's sad that I'm completely serious, because that should sound ludicrous.