Years ago race horses were slaughtered for dog food, back then it was the humane thing to do, I think it still could be. No group of Americans is going to be able to get together and take care of all the horses that would be going to slaughter. Only other thing I can think of is make the BREEDERS responsible, at least for the horses that are NOT feral. Chip each foal at birth, require the breeder keep track of that foal for the rest of it's life, and take it back if it is unwanted. Many dog breeders do that now, I've owned several dogs that I got as adults, because the dog was returned to the breeder for various reasons, the breeder kept track of all their pups, and REQUIRED the pup/dog be returned to them when the owners could not keep the dog for ANY reason.
If the slaughter of horses is about $$, then make them do it correctly, humanely. The breeding of horses is all about the $$$, nip the problem in the bud. I think it's stupid they don't geld as many feral horses as possible, or is there a 'vasectomy' procedure that would work, just like in humans. Okay, I'll get off the soap box.
One more thing, they want to do things like this, to native animals, but feral horses, oh no, don't touch them, I don't get it.