you need to consider how much 250 mg or 3 grams are. a bit of rust, enough to leave a visible red stain on your fingers, is maybe around 5 milligrams, upper estimate, it may only be 1 milligram. so you need to rub your fingers against rusty iron until they visibly have red dirt on them, and then lick them clean, and repeat that for 50 times at least, to get to 250 milligrams. Now how is a BP with its tiny toungue supposed to get that done?
rust doesnt look good and is hard to keep clean, but really, not poisonous or toxic. still thinking about a good analogy for 250 milligrams. a small bean, or a large pill, thats about the weight. you really need large surfaces that rust like crazy to get there, and then the snake needs to somehow scoop it all up and swallow it.
btw:
http://en.wikipedia.org/wiki/Water_poisoning