I'm also trying to create a free shipping coupon and wish it were built in. Since it isn't I was looking deeper into the custom shipping API, which I am using to get UPS rates to show up along side with FedEx rates.
Is it possible to get the coupon code through the shipping API? It looks like that's the one variable that is not passed through from the cart, so there's no way for me to manually give them free shipping based on a coupon code either. Please let me know if there is a way to get that variable through the shipping API. That seems like a simple enough temporary fix