Some ebook devices also have ‘backlight’―usually diffused light from the sides so it doesn't shine directly in the eyes. I think this is the crux of your idea, because currently even if you use lcd/oled with backlight, no-power eink will be severely washed out in comparison. And if eink gets good color and contrast in the future, it will then fulfill the two-mode technology.
The page on the wiki.laptop.org isn't too clear on the technology—presumably it's more complex than just LCD and eink slapped together. The Wikipedia page on the XO laptop seems to have some detail: https://en.wikipedia.org/wiki/OLPC_XO#Display