People have run Norm's http://motorbicycling.com/f23/lights...e-no-7306.html
quite successfully, I have one on my Schwinn (his LED taillight option too) & it's great! Thing is the magneto on these bikes has as random a quality control as the rest of the engine does lol All I had to do with the Schwinn tho was reduce the spark plug gap a hair, but it's a good idea to have an off switch for the light as idling/cold start might
Another option is the Sturmey Archer Dyno Hub X-FDD which supplies 6v, 3w. which I've one an old vintage bike with a huge dual-bulb headlight and taillight which sounds a bit like what you've got in mind.
Ofc it only works if I'm moving