My vet said the zinc procedure is very, very painful -- bad enough she has serious ethical concerns with anyone doing it without anesthesia. She works with lots of rescues and would be all for anything making spay/neuter easier on the dog - this procedure just isn't it.
There's (at least) two ways of doing a castration neuter: they can remove the scrotum, leaving nothing so it heals smooth, or they can leave an empty scrotum, which then shrinks up over time to almost nothing. The latter has become much more common in shelter/rescue because it heals fast with few complications. The first time I saw it done, though, I thought they'd forgotten to neuter the dog as the scrotal sack was still hanging down.