I never thought I would resort to AVR HV programming to recover chips. I probably still am not going to do it for parts actually installed in-circuit. But I had one DIP Attiny85 handy and wanted to get it going again, and no combination of SPI clocking and various other tricks got it to answer.
So I decided to set down the road to see if it were reasonable to do just this once.
I found this page, which was the most helpful. It turns out, that if you just want to do a quick-n-dirty one-off, you don't need to go to quite as much trouble.
Here's what I did:
I loaded the sketch into my Uno. D9 goes to pin 2 of the target. D10 to pin 5, D11 to pin 6, D12 to pin 7, +5 to pin 8, GND to pin 4, and then I took my bench power supply and set it for 12 volts and applied that to pin 1 (with its ground also going to pin 4).
I opened the Arduino serial monitor, entered a character in the input line and it promptly reset the fuses to the factory default. Mischief managed.