There were two types of screens that could do this; colour ones and black and white ones. The colour ones did not go opaque - they merely changed colour. The light still had to go through. If you did not have a backlight of similar power to an OHP, the screen would be unreadable.
The black and white ones were just that - they used calculator style polarizers that would either let the screen be completely opaque, or completely black. Any shades of gray they could do were still slightly transparent, and so the problem as I stated it remains. If you had one on a monitor stand, you would be able to see right through every colour except black. It would be like running with every window set to 25% transparency.
My other point, that there is no light source, is still valid. The OHP ones you speak of were designed to have a powerful, external lightsource. The ones in this mockup have no light source - they would have to generate their own. OLED technology is coming along, but cannot do what the mockups suggest.
I reckon that given 7 or 8 years this could be a possibility. In the meantime, having a white or black opaque screen in the same style would be fine for me