Would a z shaped rod work ? Tilt the M/C forward a bit and get a brake rod that goes behind the frame tube.
Or find a more modern M/C that doesn't have the brake light hydraulic switch plumbing and bolt it to the outside like you already tried.
Or, get rid of that switch, plug the hole and bolt it to the outside.
In either case use one the readily available conventional brake light switches to actuate the rear brake light.
I used to have a Geneva green RD, as well as the Blue, and wish I still had them. Oh well.