i discovered something yesterday while at the coin-op in Bolton...
not sure if all coin-ops are the same.
The nozzles on their water "guns" have a bendy section in the middle that allows you to point the stream of water where you want it. using this, i was able to put the gun under the car, bend it upwards, and to my surprise, i was able to get pretty much the entire underside of the car sprayed off. the only downside, of course, being that you can't actually see what you're doing, but assuming all coin-ops have the same type of "guns", this should be do-able, and it worked fine for me!
'88 Porsche 944 - SOLD
'90 Audi Coupe Quattro - SOLD
'07 E90 335i 6MT - SOLD
'08 E90 335i 6MT - SOLD
'13 BRZ SportTech 6MT - current