THE KNOCK SENSOR IS REALLY A COOLANT DRAIN PLUG.
GM's shop service manual, chapter 6E2-C5-1 - "Electronic Spark Control (ESC) System" does not mention anything about this. For the benefit of future generations, there should be a step 3.5 under Remove or Disconnect, namely "Prepare to swim in coolant". At least the stuff wasn't hot anymore.
Anyway, there's now a new knock sensor. Apart from some wiring harnesses, all ignition parts are new/replaced.
Since most of the coolant ended up on the driveway, I had an opportunity to flush the radiator, coolant reservoir etc as well. This actually came at a good time - there was a hideous amount of sludge in the system. Apparently I had messed up the coolant/water ratio last year as well -- the liquid had frozen during the winter and now coolant was seeping through the water pump gaskets. The pump will be cleaned and new gaskets installed, if I can manage to pull that thing out.