Hi Adrian,
It is not possible to access the EyeX Engine from a Windows Store app. Microsoft has done their best to ensure that the Windows Store environment is isolated from the underlying OS, and they have done a thorough job — not even the Kinect can get through. To be honest, Windows 8 does provide some options for connecting to custom hardware from Windows Store apps, but none that we have found attractive or useful enough to pursue. We still hope that the overly strict isolation rules will be relaxed in future versions of Windows.
On the other hand, it is possible to reach into the Windows Store environment from “below”, if your application is running with special privileges. That is what we do with our EyeX for Windows software, for example, which provides eye-gaze interaction to the entire Windows environment including Windows Store apps. My recommendation: don’t do that unless you absolutely have to.