What I have been told is that the killer buyers still have to 'clear the brand' before slaughtering any visibly branded horse. You register the brand with the Dept of Agriculture. The under-the-mane Arabic brand doesn't count. That is why I would do it, on the hope that any horse I sold or was stolen from me couldn't be slaughtered without me getting a phone call.
I don't know if they do or not. Putting a freeze brand on wouldn't hurt anything though, unless you were going to sell the horse in the future and someone objected to it.
I don't know what the cost is - seems like it's expensive through the Appaloosa Horse Club but I can't remember.
That's the thing, a visible brand is not only a deterrant but the slaughter houses aren't so quick to slaughter a horse with a brand.