If the only way for the truck to show a trailer is connected is through the brake controller, what if you try this...

Build a 7-pin to 4-pin adapter. Take the output for the trailer brake and connect it to a simple lamp. The truck will see a connection and when you apply the brakes, the output from the brake controller will light up the lamp. The lamp brightness will vary with the output from the brake controller. Maybe there is something else you ca put online instead of the lamp that can complete the circuit.