I have seen this looking device once now!
It is a steel rope, has a mushroom-shaped head, can be pulled out from the frame and fits into a lock that is mounted at the right hand side, below the pillion seat. There is a (unused) lug with two holes in it to allow mounting of the lock.
To bad I didn’t take any pics of that.
Hope to meet the guy with his LTD+sidecar at the german Z750Twin meeting in 2017.
No idea how this option could be ordered at that time.