Actually saw these in Amsterdam, massive shortage of public conveniences and a lot of people. The council drops in mobile 'pit stops' that are unisex and very public. Out in the open, no walls or doors and for men - BUT apparently ladies do use them later at night with the aid of widely available paper device similar to that mentioned earlier. Because of the fragile mortar in the old buildings they take a very dim view of people who erode it further, hefty fines and enforced.
Here's a picture from the web:
http://janeduttonutrecht.files.wordp...amsterdam2.jpg

Cheers