Light power in Germany is FIXED on these low watts. Thats why these great 12 V generator systems have great problems in getting allowed.
Also dont mix countrys up!
The only one where You will get problems for using a non certified light is Germany.
These pseudo-officials there get absolutely crazy when given any supplemental rights (over the normal inhabitant). Even when not, these guys generally love to insist on their "I AM RIGHT" point of view, even when this surely will lead them to anger and wasted time. (so much for this ;-) ).
Dont know what might happen in any other country in Europe, but even here at us in Austria (usually pretty close to the Germans) the chances of getting more than a look from a cop for using any homemade light on a bike are pretty low (and in italy, france or spain, considering the optics of cars, there does not seem to be anyone who cares whatever).
Dunno what might happen in case of an accident, though.
Bike Generators usually happen to offer current, not voltage. Usually about 0.5 A. So when You use one without regulation, it will light up a 12V 5 W bulb. Not at low speed, but from medium on You go.
how to check: measure current without load. If voltage stays at a low level from low to med. speed on, output is cut in some way.
If You have a cheap generator, there micht be 2 z-diodes inside, mounted antiparallel. Cut them out or kill them by burning them out --> get power from the machine and light up whatever.
(mine just happens to push 2 Lux V in series at 900 mA, but that is a modified lightspin) (on the fly measure, light not finished yet)