Thanks Houska! FYI I'm using Corona 4 in R20, not any of the new daily builds.
I've run some experiments, and here's what I've learned:
First of all, you are correct, in that I can just use the black material on both sides and uncheck "Seen by camera", "Seen by reflections", and "Seen by refractions". Apparently I had never tried all 3 together for some reason, or I could have saved myself a few steps.
Here's a render using your setup, which looks basically identical to the one I first posted:
I was worried it would look like this (same thing but without "Seen by reflections" and "Seen by refractions" disabled):
I decided to try a few things to test the light speckles:
No motion blur (the speckles go away!):
Larger light blocker geometry (in case the current one was so close to the headlight glass that it was leaking light when blurred - did not fix the issue sadly, and added an ugly shadow around the headlights):
So far my best guess is that it seems to be an issue with light leaking through a motion-blurred object. I tried rendering the shot with only camera or object motion blur enabled, but both were so blurred as to be useless for seeing the light speckles (the camera is turning and the car is moving).
I've uploaded my scene to the private uploader as AutumnField_HeadlightLeak_Simplified.zip- maybe you'll see something I'm missing. I stripped out the bushes and grass, but left the trees in for consistent lighting purposes. Feel free to remove them and the cloner they're in if it makes debugging easier.