I don't why handlebar bag manufactures make their bags so you can't put a head light on the bars, it's just nuts. But you do have two options to get around it. One is to mount two lights, one on each fork blade. The other is to get a Minoura SGL300 quick release accessory holder that can clamp to your handlebar and a light and or computer is clamped to the T bar, then it's rotated upward just enough to clear the bag so the light can project forward..

Never mind someone already mentioned the Minoura, but you could mount a light or lights to the fork blade if the lights bracket will accommodate an odd size shaped surface to attach to.