I agree that this is likely to make the mouth numb fairly quickly. Of course I'm really not sure of that, either. Maybe it would just evolve from a sharp pain to a dull ache.
If it did numb the area I don't fully understand the implications of that. Would the numbness then distract the horse from the foot pain? Would the lack of sensation cause such mental confusion to mask something someplace else?
Or was the application meant to just get the horse through a quick rail class and its associated inspections and then be quickly removed?
I'd have to pull and re-read the HPA Regulations to see if it's even illegal. I'd consider it a form of "stewarding" which is forbidden. A good lawyer might argue otherwise.
In any event it doesn't say anything good about the culture of the Big Lick horse.
What is the origin of the photo? I'd like to re-post it someplace where some answers might be forthcoming.