But this post has a happy ending! Perhaps it was because of walking in front of this large poster, or... because she saw me, tonight just after I took this photo I saw a woman pick up after her dog!
The sign says, "We can do the maximum, but not the impossible. The cleaning workers are cleaning behind us. Let's avoid soiling behind them." Pretty weak compared to the problem, which is that 99.9 % of dog owners in Paris never pick up.
That makes the sixth person I've [ever] seen in Paris cleaning up after their dog!