I was thinking one way to perhaps do this without digging in to the C3 codebase would be to do it via a behavior. You add the behavior to an object, and if the object is picked you can reference it's picked index in the sol something in a form like you can with the iid or Sprite(0), but instead using the behavior Sprite.myPickBehavior(0)
I don't know enough about code to say if that's even possible, but it would be pretty slick.
Then again I don't know what else the behavior could do, other than perhaps work as some class type of thing such as Solid did in Construct Classic, minus the solid physical aspects.