Mayo, I think you are in the rare position of possibly being, or at least knowing, the sort of person who could help such a horse, but I'm sure there are more out there than you or your friends could take on.
I know a lady who could have rehabilitated my dog, but she has 40 years experience, it is her life, and her career - I do not have the experience, facilities or time to do what she could have done, as I have a job and other commitments and other animals to consider, rehabbing aggressive dogs is not my life. Sadly, she can not possibly take on every aggressive dog, and in a world where animals with no problems whatsoever are killed simply through lack of homes (think of all the racehorses who don't make it every year and are slaughtered, they could have gone on to make great horses for someone, but the homes just aren't available), surely it makes more sense to find homes for the ones without issues?
If it was a choice between finding a home for a young ex-racer with no hangups or a proven dangerous horse, which do you think would be easiest? Which one should be pts? In an ideal world, neither, but as long as horses and dogs are killed every day just because there are too many of them being bred, I would pts the 'problem' ones as the others are a damn sight easier to rehome (and even then there's still not enough homes).
In the current financial climate, rescues are going to be inundated with lovely, cuddly, well-behaved ponies and horses, and there won't be enough homes for them, let alone dangerous horses.
Don't get me wrong, I am an absolute softy and love animals, and regularly cry at the plight of dogs on Dogpages, but I am also a realist, and you can't save them all, it's simply a question of numbers. Its the same when people run campaigns to raise £1000's for an operation for a rescue dog -to me that dog should be pts and the money used to help countless others - one life or many?