I think it might have been done by fitting a D/A converter to substitute the digital signals from the sidestick for the normal Concorde stick resolver output. The 'laws' could then be treated as a special case of pitch damper etc. inputs so that the standard Concorde electrical signalling system could be used downstream. I don't know this for sure, it is just a thought - maybe Christiaan could comment on its feasibility.
My only reasoning for guessing on the Emg flight system was that its would be easy to turn on and off and recover to the normal flying controls.
For a very short test programme, well ahead of any A320 programme which did not come about till 1984, I suspect this was no more than a belt and braces to see if you could fly an aircraft from a side mounted stick.