You may be confusing the controller for electric brakes, which is an entirely different animal.
A trailer equipped with electric brakes has an electromagnet in the brake drum that is held close to the "face" of the drum. The arm the magnet is on is part of the brake linkage. When electric current is passed through the electromagnet it attempts to stick to the face of the brake drum, which due to its rotation ties to pull the magnet along, which applies the brakes. The greater the current to the magnet, the more braking force applied.
To control the current to the magnet the tow vehicle has to have an electric brake controller. One of the connections for the controller is to the brake light circuit so that it can sense when to turn on the brakes. The controller also has an "inertia sensor" to establish how much braking force to apply.
None of this applies to surge brakes.
I would not expect the "reverse lockout" to be able to provide the function that you want. There are two possible ways that the lockout could work, and I am not sure if ALL systems are the same.
One way, and I believe the most common (if not only way) is that the reverse lockout is actually a bypass. When the solenoid valve is actuated (via the backup light circuit) the valve allows the brake fluid to flow freely back to the resevior. If you activated this circuit while traveling downhill, the brake coupler would collapse to its limit, pumping the brake fluid back into the resevior. If you then stepped on the brakes, the trailer brakes would be inoperative because the coupler is already at its limit.
Another possible way that the lockout could work is that the soleniod valve is in series with the brake line. If the valve was normally open then normal brake functions would work. If you closed the valve then it would stop the brakes from being applied. If you connected this system up to the brake lights then it would stop the trailer brakes from changing while your foot was off the brake. Note that I said "changing" and not "applying". If the brakes were disengaged they would stay disengaged. Unfortunately, if you stepped on the brakes, which would apply the trailer brakes, the trailer brakes would stay on once you took your foot off the brake pedal.
A great source of information on trailers, trailer brakes, etc. is the following web site: http://www.championtrailers.com/brkart.html#hydraulic_brake_basics