When I rode Ft Howes last year I don't remember not being able to go back to my trailer. Matter of fact, I know I did go to my trailer. What I didn't like was having to pull my saddle at each vet check, do the vet exam with trot out, then before going out having to go back to the vet with saddle on and do another trot out. Each hold we had was 60 minutes, but having to do that made it more like a 30 minute hold. It really cut into my time to get anything done for horse or rider.
I will admit that I did have some help from an FEI 100 miler crew a couple of times. That was luxurious. ;-)