When opening a powerup, the parent window can post a message to the iframe's window containing certain metadata, like the arc id, or really whatever you decide. The data can be available via a postMessage(), and if the powerup developer wants to use any of that data, they can add an event listener for "message." Then you can allow powerups to contain data about the story.
This would allow powerup developers to access data about the current story.
This is a great idea and would be very useful for our users.