The basic format to follow when defining your own renderable.

parser.js

var d =  new Diabox({parser : function(target){ 
  // use target string to determine the renderable key. ie 'custom_key'
  // return null if the target does not fit your criteria
}});

register.js

// d is a diabox instance
d.register_renderable('custom_key', Diabox.MyCustomRenderable);

renderable_interface.js

Diabox.MyCustomRenderable = new Class({
  Extends : Diabox.Renderable,
  render : function(){
    if(!this.retrieved()){
      var content = // utilize this.target to generate content
      this.set_content(content);
    }
  }
});