Is there a powder or liquid I can sprinkle on the cement beside my car
that, when peed on, will make a noise or billow smoke or stink or do
some other surprising thing that could deter people from using the
spot between my car and the wall as a urinal? (I don't wish to damage
my car, the cement or the wall, or to injure anyone, so both the
mystery chemical and the result of urine + the mystery chemical must
be non-toxic.)

For under $50 you can mount a fake camera on your steering wheel or
head rest. Simply post the signs on the inside of your car windows
warning people that they are being video taped.

http://www.pimall.com/nais/fakevideo.html
http://www.youcansave.com/fake.asp

I think these might do the trick.

Regards;
tutuzdad-ga

Having perpetrated this offense many times in my college days, and
especially considering the circumstances amid which I did so, I
suggest you might be overestimating a man's ability to change course
midstream. Easier perhaps to park in less inviting spaces?

Perhaps you could post a sign on the wall itself warning against urination.

Thanks for the comments. I like the fake video camera idea, but I
hesitate to tempt people to break into the car. I share a garage with
about 20 other cars in a rental community (my space can't be changed,
unfortunately). I cannot add more lights or signs on the wall. I like
the fake doggy doo idea. The real doggy doo would defeat the purpose
of trying to have a clean parking spot. The thought of stepping in
that with the kids in the mornings is just too gross. Dead cat? Ew,
and poor kitty. (Where does one find a dead cat, anyway?) I like the
porn idea, but I have a feeling the landlord would frown upon it.
Perhaps if I disguised it as graffitti... What is "olde glory"?

Here you go, this should do the trick:

http://www.chiefsupply.com/tape.phtml

Buy some police crime scene tape and put it between you car and the
wall. Maybe whoever is doing it will think twice if they believe the
police have been poking around the area or someone has recently been
assaulted there. This may be particularly effective if you also draw
an chalk outline of a body on the pavement in the area that is being
"visited" regularly.

Regards;
tutuzdad-ga