I would recommend looking at the sample controls that are provided with the SDK. There you will have working examples of most if not all of the extension class types.
In terms of your issue with the label display, there isn't an override method that you need to include. Your Open UI code has responsibility for rendering what ever you want in that control space. Just because you are extending from a ButtonDisplayAdapter doesn't mean your code can only draw a button. You can draw whatever you want. The class you are extending from simply defines the interface between Agentry and your Open UI code and what properties were set / available from Agentry.
--Bill