From the Last Frontier! So let me understand you correctly, you would like to mount a light via your bayonet lug? Never heard of any product that would do this, mainly because the bayonet lug in its self would not provide a good enough mounting surface. Look at the guys that have bayonets attached, there's usually still a good amount of wiggle room.
Now if your talking a mounting option that would go in front of the lug, then we're talking. Pretty good number of mounts out there if you're looking for something traditional or with picatinny rails. Here's a whole thread on light mounting options: Weapon Mounted Tactical lights and Lasers