IBInspectable - Exposing the controls of a parent class

I want to create a IB_DESIGNABLE UIButton class to control the roundness of its corners but as stated by this question in order to control the button roundness I must change the corners of the button's view. In that case the poster creates a category.

I want to use the idea and create a UIButton class where I can control that.

But this is my problem: in order to Interface Builder show the exposed properties I have to create a UIButton class and assign that class to the button.

UIButtons inherit from UIView. The properties I want to expose are from UIView but that has to be done from the button class.

Any way to do that?