It seems there's already a monitor which offers to upscale your 120Hz signal to 240Hz: http://www.eizo.com/global/products/duravision/fdf2405w/inde... . And the product page suggests that there's already a ready market for it, among viewers of high-res satellite imagery: apparently the blur when scrolling such images on a mere 120Hz (and high-persistence) display is notable enough to be distracting.