Let me preface this with I have NOT done it on a bike...

but, when I ran alot I used a PETZL head light that I had purchased for climbing. It's a very strong halogen light and lasts a long time. It has this head gear setup that you wear like a cap.

It's elastic and MIGHT fit over a helmet... I have to dig mine out and check. Anyway, assuming it works would this not be a reasonable alternative to bike mounted lights?

The headlamp part is on a swivel type thing so you can adjust the angle to the front... might look kind of geeky but I think it would work.