Another team member found a solution, half by accident if you believe the luck of that. It’s similar to what you’ve described. We built a shell around the tank with cutouts for windows. The shell is just a 0% opacity mask. For some reason this stops the light leak. It’s not a perfect solution but we can work around it by disabling the shell mesh based on whether the player is sticking their head out the tank.
I will try you suggestion though Leomerya, just in case it helps cut down on the overdraw slightly.