My best mate has had loads of experience getting horses up on trailers that would be something they would NEVER get on in a million years for their owner. I have rarely seen her approach the ramp, perhaps get the front end on it and then retreat so to speak. What does that exactly teach? That you only have to go so far before the ordeal is "over"? Perhaps I misread that and you mean merely make them back off the ramp and come back up and go in further... in which case I have seen happen. No means for attacking your method Spirithorse, I'm just trying to understand what you meant!
I think horses, as naturally claustrophobic animals, find its not the ramp that worries them, or actually being inside the float. It's passing that edge where the roof begins. From there in, the horse realises it can't lift it's head as high as it may want. It would make sense as to why some horses prefer the warmblood sized trailers. My 16.1hh gelding originally would look at the space and go "you think I'm going to fit in there?!" He really proved the roof theory... he had no problems with the ramp, no matter if it was more bouncier than some (unfortunately we don't have our own transport and when hiring sometimes you have to opt for an amazing floor with a ramp that's in good condition but not amazing. And he had no problem when he was inside, even if his long body was JUST in. It was the moment where his head would cross from freedom to restriction.
In one session with my best mate, he learnt to self load. Now I don't have to worry about him rushing backwards when he gets to that point on the ramp. Because he's allowed to take his own time to get passed that point and load himself. He is not rushed, and he is not left to take forever. He get there, takes a breath and gets it over and done with. There is no fear or anxiety in that reaction, he simply knows what he has to do now.
It always makes sense as to why horses will pretty much load themselves on a truck. The roof is MUCH higher than one on a trailer. The horse doesn't have that fear of hitting themselves (if they decide to lift their head) and can see that there's a lot more room height wise.
I personally like to know that my horses will trailer well. There's nothing worse than needing to go to the vet for an emergency xray or making a trip somewhere on the spur of the moment, and then having to take an hour because you know you have a time limit, and the horse doesn't want to get up.