No reason you can't. I think when undercarriage/wheel and rocker panel blasters are angled at all, they tend to be angled toward the front, so they hit the car sooner as it is driving in. Plus, if they are on a timer, or turned off once the car hits the treadle, they most likely will have covered the entire area of the car.
Might want to be careful of too severe of an angle toward the exit, as you might get some interference with your blower (if it is drive-through), and/or exiting cars.
Maybe a straight on, perpendicular target directly toward the side (no angle at all) will alleviate your problem, and not cause a new one. That is the angle that most machines use with their stock wheel/rocker blasters - not really any angle.
I have seen gatlin guns, but am not aware if they have to be angled or not when they are stationary. I think in tunnel applications, they can actually sweep, and follow the cars.
Let us know what works.