I could swear this is what urinals have been designed with for years.

Sadly it is enough to apply some basic physics to avoid that back-pee.
Of course I am not discussing it here.

I see it is only English speakers who pee against German walls ....

So the beer crew will just pee further from the walls. This is uncharacteristically not a good solution coming from Germans. They should just install troughs in the spots where most peeing activity takes place. Or lights. Men don't usually pee in lighted areas.
Now how about a paint that makes removal of graffiti much easier?

Achtung! Nicht pissen.

Pee at an angle. Problem solved.

Not a great solution.
Much better to put in more public restrooms or even outdoor urinals.
First problem is that most of them are drunk, which means confused, aggressive and with muddled vision. Many wouldn't know that the wall peed back until the next day.

why not add more public urinals?

Will this force pee-soaked visitors to take their liquid ejecta with them say on the subways, buses, and taxis to other parts of the city? That's much more appealing.
A more entertaining solution would be to mildly electrify the walls. Perhaps city issued catheters with bags?
On a serious note, everyone knows, when you gotta go, you gotta go, and no amount of dissuasion will make a lick of difference. As suggested earlier, plentiful access to urinals is the key.

The grown-up word is urinate.