Parking a 3-speed internal hub in 2nd gear permanently
In the 45 minutes since I posted this question, I have developed an elegant workaround using a simple device:



I stuck a pebble in the bellcrank.

Most folks here probably won't be surprised by this solution, but I'll elaborate on how I came to it for the sake of future searchers who might be internal hub noobs like me.

In fiddling with the push rod, I determined that since I have three gears, it probably has three positions: all the way in, all the way out, and... somewhere in between. I took a wild guess that second gear would be the "somewhere in between" gear. I figured that "all the way out" was third gear, since that's the natural position the rod is in when there's nothing in place to push it anywhere else, and 3rd is the gear I was stuck in.

I fiddled with the bellcrank some more - putting it back together, seeing what pieces would push on what if it were working properly - and determined that when the bike is in third gear, the head of the push rod butts right up against the inside of the part of the bellcrank that screws onto the threaded axle rod thingy. (Terminology is hard.) So: I need a way to push the rod into the halfway position while the bellcrank is screwed on in order to jam the system into second gear.

Enter the pebble. I found one in my yard that seemed to be the right size, dropped it into the bellcrank and put it back on the bike. Voila: the push rod is stuck in the "somewhere in the middle" position. I am effectively parked permanently in second gear. (Well, I can also shift into third gear if needed: I just have to stop, dismount, and unscrew the bellcrank a little. If I had a second pebble of equal size I could probably get myself into first gear, too!)

So, this is my workable, cheap (free!) solution. I will leave this question open for a while though because I am hoping someone has something a little less bush-league to suggest!