OldMac and I had a rather detailed conversation one day regarding using the little halogen lights along with traditional lights. It turns out that there is a large spike of the deep red in their spectrum and that they are very effective as "trigger" lights, used for 30 minutes at the beginning and end of the light period to simulate sunrise and sunset to reduce stretching.
I would be very curious to see what was in the "white" spectrum of these headlights and how much of it was in the effective grow ranges and try to guess what effects they would have in the normal growth cycle of a plant. I am afraid that upon careful inspection you would find that as far as the plant was concerned, a normal grow light would be more efficient than a light designed to be brightest to the human eye. As Shovelhandle said, someday someone will try it and let us know what they find out.