My main question is "why is side visibility so important?" If you're riding on a road and you have a good tail light and headlight, you shouldn't have side impacts unless you're running stop lights or stop signs or something.
However, there are some options.
Scotchlite tape is a fantastic thing, though passive (but with a decent range of angles with visibility).
I'd say use one of the multitude of tail lights, reflectors in your wheels, reflective sidewall tires, or one of the many side lighting systems that are now being sold may be useful. Do note that you shouldn't have enough lighting on the sides to be distracting (like those people in the second picture).