The side stick fitted to 201 (in 1977 according to Flight International), was very much proof of concept for what would be the future A320 programme sponsored by the French government, very very limited hours were flown using it apparently (like 10!). The next step I believe in the development was to fit it onto a FBW modified A300 in the early 80s, but for the initial tests to see if a pilot could fly with a "computer joystick" it had to be done on Concorde as this was the only a/c with a suitable FBW system.
It would be good to know how it was done, but I guess it could have been wired into the emergency flight control system, which relied on electrical strain gauge inputs to move the control surfaces if there was a control column jam.