I found my mother! This time, earlier this evening, I could mostly - although not always - tell her apart from the columns. There was no acqua alta to confuse the issue or to make the piazza more lovely. She helped me out by phoning me when she knew she could be seen by the webcam. This particular screen cap shows her drawing the piazza. Here's the drawing she was making! She's about halfway down the colonnade, on the left, standing in the middle of a well-lit archway.
(See here for details of earlier, unsuccessful mother-spotting attempt.)