Aha! After many minutes of searching the technical manual I found where the "coolant guide housing" contraption is discussed. The line *should* come off with the standard "push the plastic ring in and pull the line out" procedure that you mention. (Step 1 in the diagram.) Sounds similar to detaching the power steering line coming off the back of the PS unit, which I have done twice recently. I'll give it a try. This should allow me to simply insert the stopper plug in place of the vent line.
