Luckily we do not have that problem here.
I also wash my car well off the street almost in my back yard.
Out of curiosity how do you currently clean your car ?
I assume that your "hand wash" sites have high pressure hand wands of a type similar to this home high pressure cleaner http://www.justtools.com.au/images/k200_plus.jpg
These can be used to clean car paintwork but just keep your distance so that the water pressure is very low on the paintwork so as to reduce the chance of dirt being forced across paint surface. ie just use the wand as a mist generater
If you can first use one of these to wet the surface down and then use the normal bucket technique followed by removal of detergent with a mist spray from the high pressure wand then should be OK.
If the Wand sprays the detergent as well then as long as you keep the water pressure low at point of contact with car paint then you could mist and wipe down with your "sponge".
Might take 2 people - one to mist car and the other to "sponge" paint down. Could be nice on a day
Not so good on a cold day eek!
Remember that High pressure will be OK if only "mist" the water on paintwork. Might need wand a meter or so away from car for this senario.
Your problem will be what to do once you have a clean car.
If you can polish at home then you still have to drive home.
If it is a nice day then you could drive home and use a "dry" cleaner to remove any dust etc on the car prior to the polishing.
Meguiars Instant detailer, for example, will do the job nicely if cannot just wash with water.
Just spray the Instant detailer( or equivalent )liberally on car and remove with terry toweling cloth. Then polish your car.
A friend who is an Airline pilot used to comment about Germany's tight regulations.
He then would tell how many times he had dumped full loads of fuel from his aircraft when doing emergency landings. frown