I'm running 24v for my headlight/taillight/brake light. I just shunted 24v off the 48v battery pack. not ideal but it works.
I recently bought one of these off of E-bay. supposedly can be run off of 24v - 90v.http://www.ebay.com/itm/12w-led-head-li ... 231ba5077c
but I don't expect delivery for a few more weeks. i intend to use in on a bike I am building which primarily will be run on 48v but occasionally 72v (or vica-versa? not sure how the motor is going to perform that 72v yet).
I'm also currently looking for a switch-mode regulator to supply power to a 36v lighting system from a 48v battery. haven't been able to locate one yet so I was thinking of using an old 48v brushed motor controller and using a potentiometer (or resistor?) to permanently set the throttle at 75% or whatever so the output is around 36v. not sure it will work but i will give it a try.
also, since they are cheap and I have an old bike light housing that will hold 2 of these 24v halogen bulbs
I was toying with the idea of wiring them in series with maybe a capacitor to smooth out the initial voltage spike.