I'm glad the toad didn't harm your pac, but you're looking at potential harm from toxicity the wrong way. All the animals you mentioned that eat Bufo americanus evolved alongside them, and have systems that we KNOW can handle that type of toxin.

The toxicity of Bufo marinus isn't what leads to it being such a problem in Australia. There are dozens of snakes, lizards, and fish in its native habitat that can eat it an suffer no negative effects because they evolved ALONGSIDE it. Saying that it is a problem there because it is so much more toxic than Bufo americanus is to ignore the fact that it isn't potency, but evolutionary relationships that determine danger to predators.

Another example that I have seen personally involves garter snakes (which I raise). These are known to eat many amphibians, and are the only known animal that can safely consume fire newts. However, they cannot eat anything that has touched my pacman frog (I would occasionally offer them a pinkie she had refused). It will result in a regurge within the hour. This is a snake that can safely consume one of the most toxic amphibians on the planet, and they have adverse reactions to the toxins a pac produces, which are so mild humans don't even consider them an irritant (and why I didn't think anything about offering them something that had touched her).

Also, once again, I'm really glad for you and your frogs that this seems to work but promoting this strategy as a safe and effective way to 'experiment' with new food sources is just... incorrect. Unless you've done a detailed comparison between the proteins Bufo americaus produces and the proteins produced by ranids in the pacman's natural diet and found them to be highly similar, deciding to feed the toads to something outside their natural range is no more well-researched than tossing any random animal into the pac's tank and hoping it doesn't have adverse effects, since (to my knowledge, please correct me if I'm wrong) there is nothing in the actual literature regarding Bufo americanus as a feeder for horned frogs.
I respect your experience in husbandry (seriously, I do), but unless you have information you haven't shared with us, you just got lucky, because the 'methods' you described for determining this would be a safe feeder just don't add up.

Sorry if I sound harsh, but this is kind of a pet peeve of mine... I totally understand that sometimes unsafe/unknown feeders are a necessary last resort. It's just that a blind stab at safe feeders shouldn't be touted as a safe and well-thought-out way to determine if something is a good feeder or not for hobbyists.