Can you post a small example of this. The scene would need to load without support of any of Unity Gui components.
U3DXT does not expose all of the UIKit framework yet. We support a few elements like UILabel, Picker, Alerts, and WebView at the moment. We do not have support for UIButton yet.
For a full list of which classes we currently support in UIKit, please take a look at the UIKit Framework API.