I think you are right on this issue. That way the trailer is not activating the brakes when I do not want them to be working. I do have a lock out pin that I insert into the tongue for backing up, homemade and very cheap.
I still would foresee a downhill issue also. The only 2 differences I knew of,(disc vs. drum) reverse solenoid and higher pressure (disc is higher). You may want to look at adding an electro/hydraulic actuator instead. Not a cheap alternative by any means, but ........