There are sprayers for cleaning leaves out of house gutters that have sort of a shepherd's crook with a small sprayer at 90 degrees.Could probably lay it flat on the ground, sprayer pointing up, to remove under-carriage debris.
If you use much more than mains pressure expect water inside the car. I have an under body wand for my pressure washer with 60deg upturn & swivelling fan nozzle for cleaning under wheel arches. I do not use it on my M3W.
Tim
