The latest Beta version is available here:
However, when I tried to disable my NVidia discrete graphics card on a rMBP 15" running Windows TP in Boot Camp, the Intel Graphics chip wasn't listed under Device Management upon restarting, so I wasn't able to install the HD Graphics driver manually. I don't have Windows 8.1 installed anywhere (I prefer Windows 7) so I can't test this under that operating system. But unless the integrated graphics device is listed somewhere else in Device Management it doesn't seem to be recognized in Windows TP even after disabling the discrete graphics device.
Cheers