dsc810,
I think the pilot would have been prepared for any sudden position change, following the closing of the brakes. However, my comment is not meant to minimize the risk of a tow-plane upset. When I'm towing, below 300', my fingers are poised on the release handle.
I used to instruct in and tow Blaniks. The airbrakes would always come open during takeoff if they weren't locked. Once I was towing an inexperienced pilot in a non-radio Blanik and the airbrakes came open during the takeoff roll. I was climbing at less than 200' per minute.
I was reluctant to give the rudder-waggle signal for "airbrakes open", because the roll secondary-effect could be mistaken for the "release now" signal.
So my plan A was to try to turn and climb to a suitable release location in the circuit, which I was able to do. Plan B was to continue straight ahead and release him over a nearby airport.
Taking to the pilot afterwards was very illuminating. Until he released, he had no idea that the airbrakes were open and he assumed that the low rate of climb was due to engine problems. He told me that his hand was on the release, anticipating a signal from me!
Talking to another tow pilot about my experience, he told me of a similar experience, where he had given the "airbrakes open" signal, which the glider pilot mis-interpreted, released, got too slow and spun-in.
I decided that I would never use the rudder-waggle signal unless in absolutely dire straits. Of course these days, we have radio, so the problem has gone away. However, we still teach all the signals and the way to minimize mistakes, is to do a very quick rudder-doublet.