Peter White explains that modern lights have voltage limiters in them. Still, I wouldn't use a generator that is rated to put out 12V.

So I gather (and dimly remember) that the bottle dynamos always put out 3W and the dynohubs had lower output and lower drag. Now they have higher output, so the big disadvantage with bottle dynamos is that they present more drag than hubs. Still, the higher drag is sometimes justified by the greater convenience.

I now have three bottle dynamos to play with: a Union, made in Germany, marked Schwinn-approved; a Miller, made in Great Britain (and gorgeous), and one marked "Bike Parts." Here are the last two which I got cheap on ebay. I got the Schwinn one off an old bike. The Miller and the Bike Parts generators appear to be intended to mount on the drive-side chainstay, since they're supposed to be forward of the mounting point. Not sure why the tail light is mounted as it is. It is also branded Miller, but it's made in Germany.