Out of curiosity (not criticizing, just wondering) why do you want to go the dynamo hub route instead of a simple battery-powered light such as this one?

This LED light is bright (1000 candlepower) and has good battery life (70 hours) on readily available AA rechargeable batteries. You don't have to deal with the hassle of building a new dynamo hub wheel, or with the extra rotating mass, or with the wires running all over your bike, and it is a small fraction of the cost of a dynamo setup. At the price you can easily afford to put two of them on your handlebars, one aimed close the other for distance.

I have a similar light on my road bike (can't see it very well in the picture) but I have been quite satisfied with it. Sadly, I have not been able to ride my road bike very much on the cobblestone streets here...

tanstaafl.