Thank you all for the terrific responses. I hadn't seen in my mind how you could steer a boat without some kind of a rudder until reading your responses, and I really like the "drag something" solution. I just hadn't thought of that. That solution of dragging an anchor and using winches to move the temporary "rudder" to steer, that's fantastic! I guess the key would be getting the amount of drag right for the speed of the boat, that you would have to adjust it from time to time while you were moving.
Thanks again! It gives me some hope that I might be able to come up with a solution if I got into trouble, so long as I had enough line, mattresses, etc, on board!
But of course that would be like ... not even a backup plan, but rather the desperation fallback. I'll try to get some kind of a spare type solution with the windvane gear.