Bit of a crazy solution but maybe you could take your steel shaft, cut it in half along the length (leaving two semicircular sections) then stick/bolt the two halfs back together with a thin layer of insulating plastic between them. The top of the axle is open at the top, you then have a circular magnet which you drop on the top of the shaft and acts as a conductor so connects the 2 halfs of the shaft. Inside the robot you have two "brushes" like on a brushed motor pressing against either side if the shaft, this acts as your link. To activate you put the magnet in place to deactivate you just pull it off.