Watering the plants outside ceased to be an option when the weather turned cold about a month ago, so since then I've been watering them in the kitchen sink. It's warmer than outside, but has a much smaller capacity, so watering has been taking a long time. (My best guess is that the last round took about twelve hours in all, spread out over four days.)
So the logical solution, and in fact the plan all along, is to put a shower/tub in the plant room. It's not going to be the solution for everything. (I'm unsure, for example, whether the really heavy plants, like the Big Damn Screw Pine, or the Murraya paniculata in the thirteen-inch clay pot, are going to be able to sit in the tub when wet without bringing the whole thing down: the heavy plants are very heavy indeed. Out of curiousity, I just tried to weigh my fifteen-inch Cordyline fruticosa on the bathroom scale, the scale says it weighs 46 pounds. Which is not as much as I was expecting, truth be told, but is still plenty awkward to carry up and down the stairs.) But I'm hoping it makes my life either a little easier, or that it will leave my life just as difficult but facilitate the addition of more plants. Whichever.