I've tried aluminum foil over a lens as a cutoff. Walking a distance from the bike, and observing it in person, I literally couldn't tell the difference from having it right-side-up or upside-down. Nice concept, looks good when projected on a wall, doesn't help IRL.

That seems counterproductive. You don't want your headlight to be invisible to traffic in your front 180° arc. At least, *I* don't