I live in the Colorado mountains so this is an important topic.
The conventional wisdom here is that "surge brakes don't work in the mountains".
I have looked into the idea of a stronger spring in the actuator - as suggested by desertdeals. I can't find any manufacturer that says that is even possible. You would certainly be on your own and the spring rating would be a toss of the dice.
I have thought of another solution that I am considering trying. This assumes that your actuator has a reversing solenoid switch that can lock out the brakes when backing, and assuming it is the type that works with backflow to the master cylinder.
One could wire a control for the solenoid that allows manual control in addition to the backup light control. Then when using engine braking on long hills just turn the trailer brakes off (locked out). You would need to be careful to remember to unlock them again and so this has its dangers.
Or... you could wire it so that the brakes can be manually locked out .BUT. they would be unlocked again if the tow vehicle brake lights come on. So on long hills, you lock out the brakes but if you hit the tow vehicle brakes, then the trailer brakes would be re-enabled. (over ride the manual lockout). This would be simple to wire.
In fact, with this solution, you could wire it up so the brakes are always locked out except when the tow vehicle brakes are applied. In other words, engine braking would not cause the trailer brakes to work at all.
The one thing I don't know - how do the trailer brakes behave when locked out and being towed forward ? Does the actuator bang back and forth loosely ? The lock out is designed for backing up - not forward travel.