The Developer behind the popular TinyUmbrella program is now said to be working on a new tool dubbed TinyPwn which will allow you to create and customize the iOS custom IPSW files.
TinyPwn: Mac/Win/Linux ipsw customization tool. Will allow scriptable modification of the ipsw via a simple custom API. (and more )
We don’t have much details on it yet, except the fact that it will be supported across multiple platforms (Windows, Mac and Linux).
More on it as it comes, stay tuned for that !