My understanding is that the nVidia GPU doesn't actually have access to the screen; when you turn on Optimus mode, the nVidia GPU draws its output into a textur

My understanding is that the nVidia GPU doesn't actually have access to the screen; when you turn on Optimus mode, the nVidia GPU draws its output into a texture or frame-buffer on the Intel GPU, which actually pushes the pixels out to the display. So if you could disable the Intel GPU from the BIOS, you'd wind up with no video output at all.