I have used this technique before and it workds great. My question is... how long do you think they will hold up? I was thinking about using this method to make animals for a Noah's Ark cake. I want to place toothpicks in them and stand them up. Do you think it will work?

No, as soon as they come to room temp they will be a pile of icing. You could do color flow or the same technique with royal icing.