In practice that hasn't ever been a concern. Every pup I've done this with goes from a mad fierezy to ginger nibbling.
They're smart enough to realize there are now non-edible things in their bowl. The stones are much larger than the kibble, too large to really fit into the mouth much (wide, flat, smooth)
I do monitor them closely. I suppose if I didn't get the desired outcome with another dog, I'd try a different approach.
One can also train a pup to use just one part of the yard for a toilet. Dogs be smart an sheeit.