JDI, don't you think if the horse knows he has a tie-down on he will not even attempt a rear? Assuming, of course, that the horse has basic training.
In my situation, it wasn't a rear, it was simply a head throw, as the OP described. I don't recall her feet coming off the floor of the trailer, though I could be wrong. What I needed to do was keep that head down, not prevent rearing. That nightmare co$t me an after hours vet trip.
Those helmets and pads really don't seem enough to protect the horse. Looking at the fly protector in your avatar, that would be good if it extended another 3 or 4 inches back, but I've never seen a bumper pad like that.