I stopped by the local dealer and picked up a screw-in heater. The instructions for installation in the box were totally bogus in that they involved cutting a hose. A dealership service guy took one look and told me to destroy them. Then he took me out to a new B-series on the lot, swept the snow off the hood, and showed me how to unbolt the large Allen wrench and quickly twist the plug in before too much coolant runs out, taking care to treat the new threads with a sealant before completely removing the plug. He told me it takes two minutes to do the job.
We'll see.