I've never heard of anyone doing this. And while it sounds like a cool idea, I'm not entirely sure it would work. Usually grafting is done with a finicky cactus because it is prone to rot from watering or has a weak root system. I've never heard of someone trying grafting with a non-cacti succulent species. The problem with lithops is that most of the plant's structure lies underground. This is both a defense mechanism against "mechanical" damage and a way of conserving water during a drought. If that much of the plant were exposed above the graft host it might not survive.