The first parameter to the TouchTarget constructor is an IntPtr handle to an application window. If you pass in a handle to a window running in a different process, you'll get an error. This originally led me to believe that in order to detect touch input, you had to have a windowed application, and there was no option to detect touches occurring outside that window.
Fortunately, I was incorrect.
To use the whole screen as your TouchTarget, simply pass in IntPtr.Zero as your window handle, like so:
new TouchTarget(IntPtr::Zero, Core::EventThreadChoice::OnBackgroundThread );
If you do this, keep in mind that TouchPoints you receive will have x and y coordinates relative to the screen, not your application window.