However, you cannot simply hang your headlight upside-down from the rack. Good lights like the Edelux have optics that project more light down onto the road than upward toward the sky. Turning them upside down inverts the beam pattern: Now you would shine more light into the sky and into the eyes of oncoming traffic than onto the road!Together with Schmidt Maschinenbau, we have developed “upside-down” Edelux for “hanging” attachments. (You have seen prototypes on the photos of our own bikes.) Now there is a production version of the “hanging” Edelux available. We just received our first shipment.
see here
http://janheine.wordpress.com/2012/01/09/edelux-headlights-for-hanging-attachments/
$200 here
http://www.rivbike.com/Schmidt-EDeluxe-Dyno-Headlight-p/ltd-13.htm