You could set up a swing arm( think manual car wash) bolted to the side of the horse trailer that can be secured to the trailer during travel. Attach a stretchable rope from the arm to connect to the halter so there is some give in the event of a light spook. Maybe even a spring system on the arm where it attaches to the trailer then run the rope through eyes like a fishing pole out the end and down to the halter. You could even make the arm where it has several sections and can extend out to make for more room to move about for the horse. Maybe even add a hatchback/hood shock from the arm down to the base for more absorption in the event of them trying to pull free.
Hmm, I may be onto something and will head for the drawing board to see what I can famribicate...hehehe