It has been done before, kinda. The setups I've seen have the light moving the distance over the tank in just a minute or two....then reveresed direction. It would go back and forth all day long. When Alex owned Wolf Reef, he had this type of setup on his frag tank.

Now you might be able to get a slower moving motor, but I'm not sure. If you can get ahold of a few metal halide setups fairly cheap (or if you have a few already), you can always just use timers to turn them on/off in sequential order as the 'sun' moves across your tank. That way you technically only run 1 halide over the 8 hours a day.