Oh my god I don't care HOW many they've done I wouldn't let them put a single hand on either of my horses! I can't IMAGINE the pain! Anything strong enough to prevent the horse from feeling pain is NOT going to be available over the counter. And the scrotum is one of the most sensitive body areas, how can you think about letting them cut that open without anesthesia, without the proper equipment if something goes wrong? Put yourself in that situation. Would you allow them to cut open your sensitive areas with just a bit of over the counter drowser? Would you let them do it on your husband for a vasectomy? If the answer is no why would you inflict it on a horse?
I hope that once you see your horse thrashing in agony you'll second guess doing this again. Poor, poor horse.