I also remember that at least some of the RD range in the late 70's / early 80's had this type of indicator.
The CB-1 (NC27) I owned also had them, and there were no clever electronics to control it.
I think everything was cleverly mechanical within the switch.
However the switch on these bikes had no integral mount for the clutch lever, and the connectors would doubtless have been different.
So you could do something if you wanted to Greebo, it just depends how much trouble you want to go.