Connect one rod from the brake switch mechanism to the rearset, and then another rod back to the brake drum, from the rearset. Make the rearset the mid-point.
I'm sure there are 100 better ways to do this, but this struck me immediately when looking at your drawing.
My XS650 rearsets use the existing splined brake lever mount, which activates both the light switch and attaches to the rod back to the drum. You cut the brake pedal arm down, mount it at the right angle, and the rearset attaches to it with a rod.
What bike is this?