Interesting to read this. Never had a horse who would not eat some hay in the trailer! They also drink in the trailer, some better than others, but they are getting hydrated. We have done some rather long hauls, 12 hours and more. A non-eating, non-drinking horse would have me quite worried about impaction colic! We did have one new-to-us horse who refused to pee during almost 15 hours in the trailer. Turned out horse liked to stretch waayyyy out to pee, could not do that in the trailer. But unloaded, put in a stall, he went forever!
You may have tried this, but make hay in the trailer be breakfast. Horse goes in trailer hungry, might be more willing to eat then. This can be done with truck and trailer sitting in the barnyard, not actually going anyplace. Perhaps not moving trailer might help horse choose to eat a little while parked for an hour or so.
We do this loading and feeding with our youngsters, so they associate trailer with food. There is ALWAYS something to eat in the trailer. Rides on the road come later and spending longer times in the trailer. Rides with a horse friend along, then by themselves. Waiting quietly when the truck is turned off. May unload or we may not, horse needs to stand quietly when in the trailer. Just another phase of skills a well-trained horse needs to learn well.
We do let horses learn to manage a smaller hole (2 inch) hay net, get over being frustrated with only getting small servings while at home in the stall. I do anchor the nets at both ends so horse can push nose in, net stops and he gets some hay. Too frustrating for horse to push net and have it swing away!! He gets no hay at all that way. Nets anchored at two ends in the trailer as well. I want horse who is not hungry or angry when we arrive!