You can manually change the class of the cells in IB.
For example, to put a button on a HUD window
- Drag a normal push button from the object library
- Click, pause, click the button to select the button cell inside the button control
- In the identity inspect, change the class the BGHUDButtonCell.
It will still look like a regular push button in IB but when you run the app you will get the look provided by BGHUDAppKit.
You also won't be able set any extra properties in IB beyond those provided by Cocoa. You'll need to set those in code.