Success at last ... or at least another way that worked. I don't have the humidifier tied into the fan control; instead, I installed a sail switch (basically a plastic bag kept rigid by a wire that, when air is racing through the return duct, changes position and completes the circuit) and we have a working humidifier.
I'll probably keep checking it tonight to make sure there aren't any leaks, and I need to replace the drain line (the old one works for now, but it's got the detritus of the years in it and I'm sure it'll clog or something at some point. The kit did include 10 feet of drainage tube, but that's not long enough if I want to follow the old drainage tube's run (which isn't efficient, but it is out of the way).
I might try to tap into an existing drainpipe or something (right now it dumps into the sink), but I'm not sure. The closest place to use would be the same pipe that drains the AC's condenser coil to the outside, but I (thankfully) realized that that cool water draining outside during the winter — when it's likely to be below freezing at least some of the time — would be a bad idea.
Tomorrow, I'll probably play with the drainage setup and try to clean up the phone wiring upstairs ... with all that humidity, it seems like a good time for a project or two.
[UPDATE: Okay, a new short temporary drain line is now in place: There was some nasty crud in the old line, and it was backing up. I caught it just in time. The compression fitting at the saddle valve needed another twist or two with the pliers; there was a small bead of water dripping from it. Hopefully that's the last of the excitement and the house will start getting less dry.
Actually, I'm a little curious about how much water flows through the drain line; the more humid air should make the house feel warmer, and maybe we can set the thermostat a little lower (don't tell Evelin ...). At the same time, we're going to get at least a little bump in the water bill. Maybe I'll try to measure how much water is flowing out during the typical furnace cycle ...]