question

Well, there's nothing special about a Card from the skills kit point of view, it's just a JSON string (at least, the types we have access to right now), so in that context you could have your service send it out to anything that accepts push notifications. I think your question is whether there's a built in way to have it notify non-Amazon apps, though. There's not a way to do that, to the best of my knowledge. On Android you may be able to intercept incoming pushes using something like Tasker, but that would be non-trivial.