When I pulled up the plant last evening, I noticed that its root system was not nearly as developed as I had expected for such a large plant. I don't really know what is normal for a mature tomato plant, but the Better Boy's roots stretched out no more than one foot across. They subsequently had left lots of dirt unused in their whisky barrel home. Maybe poor root development accounted in part for the plant's lackluster performance.
I replaced the Better Boy with a Mortgage Lifter variety that I picked up at local nursery. The plant obviously had been there a while, as it looked pretty rough around the edges and was discounted from an original price of $4.87 to $1.85. But after pruning off the dead limbs and leaves, the thing looks good for a rescue plant. We'll see how it grows, and if it produces more than the barrel's previous occupant.