Yes, subclassing the Content class is what you need. You have two options to do this:
1- Create your content programmatically.
2- Set the default class for the “image_button” content type (used for buttons from the content creator) as your own custom class (http://catchoom.com/wp-content/themes/catchoom/sdk-ar/ios/interface_craft_a_r_item_a_r.html#a473c9d9a49da4c266b183387f1a6fbb6). This is a static method to which you should pass the content type identifier, and the class for your custom content.