The safest albeit expensive way, is with the hydraulic unit engaged with full clutch depression. You could probably wire a normally open electric brake light switch, mounted so that it completes the circuit with the clutch engaged.
With just a solenoid, it would only take one time of forgetting to depress the clutch to potentially ruining the Columbia.
Glenn
PS I have a '32 with '40 Columbia with hydraulic unit in my roadster.