Is it possible to create a game in Unity and then use for example XCode (or other handheld device SDK) to handle the GUI and server connections. Can I execute code in iOS from inside the Unity game?
This is the basic content
UNITY:
iOS:
Hi,
StartScreen: You simply set it in the player settings inside Unity, but you can also change it afterwards directly in the xCode project generated when publishing.
PauseScreen will need to be implemented within Unity. If you want it to be handled natively, I think will have to stop the unity game, and then restart it ( correct me on that if I am wrong).
GUI: inside the game, you will have to use Unity to build your GUI. But you can modify the starting procedure in the xCode project and show something else before you actually launch the unity game.
HighScore: if you want to show highscore during the game it will need to be done with Unity,
As far as executing code in IOs form inside the unity game, yes it's possible, Check this fully working example in the resource section ( pick NativeCodeExample.zip )
http://unity3d.com/support/resources/example-projects/iphone-examples
Also, check this:
http://technology.blurst.com/a-cocoa-based-frontend-for-unity-iphone-applications/
This is quit hold and some features are now built in now, but it explains a lot of what I think you want to achieve.
Bye,
Jean