The good news is that my plan B worked splendidly. At the time, I had all this caulk leftover in the tube, so I used some of it to make a silicone lip on the wall a couple of inches below the waste pipe. The idea was that if water were to get through the sealant, rather than running straight down the wall to the floor, it would be directed outwards so that it could then drip into a bucket below the lip. Damned if it didn't work! There was drippage into the bucket, and not so much as a damp spot on the floor.
I can still improve it, I think. I've been looking for a tall, rectangular wastebasket to catch the water, rather than the current circular bucket. That way more of the containment device can be snugged up to the wall directly under the entire length of the lip, with the requisite increase in water-off-the-lip catching efficiency. I've also thought of putting a long piece of acrylic sheet in the container, and leaning it up against the wall just below the lip to direct the water into the container.
