Ok, I know this sounds way out, but there are gases such as carbon dioxide and hydrogen chloride that when hit by IR light causes the molecules of the gas to vibrate and essentially absorb IR light. I don't know the degree of absorption when IR travels through such gases and the electric vector interacts with the molecules. I really don't know if you need 2 inches or 2 miles of pure CO2 to absorb the beam of a laser gun.
It appears to me that it is not the plastic or glass lens of the headlamp that is the problem, but rather the reflection from the reflector behind the lens. Otherwise the police could just shoot at your windshield as a large easy target right? I think most of the IR light passes through the lens and is then reflected back out. So if you could fill the light housing with an IR absorbing gas maybe it would work. Then again, maybe it is late and I am just getting a bit too creative with my imagination.