The trick, I discovered, was to remove the manufacturing debris from the drain! I messed with it for quite a while and one of the times I pulled the linkage and plunger out of the overflow pipe, a 3/4" long piece of plastic was stuck to the plunger. It looks like dried polyurethane and is the same reddish color that appears at one of the factory-assembled connections. After that I was able to adjust it easily.
I learned something new today: tub drains are designed to leak at the rate of one gallon per hour.
I hope this information helps someone else out in the future.