Google Plus One button callback - Any way to 'subscribe' to the +1 action?

Question

I'm looking for a way to subscribe to the plus one button.

According to the documentation here: https://developers.google.com/+/plugins/+1button/#plusonetag-parameters I could add a callback attribute to the tag, but in my case I'm not allowed to interfere. I'm building a tool ontop of the site, an embedded JS triggered on document ready. I want to add the callback live, and it mustn't interfere the original callback if one was declared.

I don't have this problem with Facebook or Twitter (like and tweet, for instance). In these cases there's the FB and twttr global variables, registered like so once they are available:

FB.Event.subscribe("edge.create", function(e) {
  console.log(e);
})

or twitter's twttr.events.bind ...

Am I missing anything or is Google choosing a very awkward way to do things? What's their interest in this method and what can be done around it?