In another thread another poster suggested a good idea. Just put amber blinky lights on the ends of the handlebars. Properly set-up they could be seen for over a 180-degree arc from back front and side with them only not being seen on the off side. Mounted as far apart as your handlebars are wide so its obvious which side they are on.

With narrower handlebars like the road bike drop bars it wouldn't probably be the best plan but on bikes with mountain bike type straight bars would probably work like a charm. I'm thinking bright 2+ watt amber LED encased in an armored plug in end piece that plugs into the hollow end of the bars with the top armor plate serving to keep the flash from going up into the riders eyes to mess with your vision at night but allowing it to be seen by others from the back, side, or front. Combined with a middle headlight(s) and middle rear tail-light would probably do the trick nicely, plus you don't have to run wires very far if you mount them on the bar ends and put the controller on the bars.