This sounds like a fun project. What I would do is get a pair of Cree LED emmiters ( http://dealextreme.com/details.dx/sku.2394 ) and hook them up to a pre-made regulator board ( http://dealextreme.com/details.dx/sku.3256 ) designed for LEDs, and hook that to 4-6 "AA" battery cells stashed in a battery holder inside the bars. Install the LED emitters into a some aluminum reflector ( http://dealextreme.com/details.dx/sku.3257 ) and JB Weld those puppies into a copper 90* elbow fitting, like for plumbing, and mount the elbow fittings somehow (so its easily removable) on the ends of the bullhorn so they face down. Wire it all up, test it, and stash all the AA cells, the regulator, and wiring inside the bars. Add a switch and you're good to go.

Check these guys out:

http://myfwyc.org/bikeled/DIY_LED_Bi...ing_Guide.html
http://myfwyc.org/Timberlake/CreeLEDLight
http://www.candlepowerforums.com/vb/...d.php?t=171702
http://www.candlepowerforums.com/vb/...d.php?t=126276

I should try this out...

BTW with those CREE emitters, you'll be pumping out some SERIOUS lumens, these aren't just decorative, a rig like this could easily compete with one of the $400 DiNotte lighting systems. WAY bright.