In the context I am thinking, I would essentially be mixing the droppings into the topsoil in a plant bed.
How much would be too much? How much would be too little to make a difference?
Is Salmonella a risk?
Frogs and toads may have salmonella in their feces, I discovered. Therefore, because chickens may also, it stands to reason that you should treat the frog or toad droppings like chicken droppings (meaning, you should compost them in a manner where the compost gets hot enough to kill the bacteria). Adding it directly to the soil is probably risky.
I know I've had eggshells added directly to soil before, and I didn't get salmonella from the tomatoes that grew in it the same year. Maybe I was just lucky, though.
I don't suppose it would be easy to determine how much uncomposted frog/toad manure would be safe. Generally, you should compost any manure well before adding it to your garden, no matter what animal it comes from. I'm curious about grasshopper manure, though. It's kind of there anyway. :) Arguably, so is toad manure (but not in raised beds).
The fact that toad droppings are dry isn't a consolation, as I was hoping, seeing as dry pet foods can be contaminated with salmonella. Freezing doesn't kill salmonella, either.
I'm guessing toad manure (not sure about frog) would add more calcium to your soil, due to all the bug exoskeletons. Other than that, I'm not sure.
As a side-note, blue and UV/black lights attract bugs, and because of that toads can like the lights. So, if you want to keep your toads well-fed, and don't mind lots of moths, that's a consideration.