Unfortunately, it's a bit more complicated. Wow Heroes uses information provided by the Armory in order to calculate an overall gear score. This information is more detailed than the information that can be obtained in-game. I really don't foresee a Wow Heroes add-on since Armory information would not be available to the add-on.
However, if Wow Heroes were to publish their scoring system under a public license of some sort, then it would be possible to more closely approximate the same score in an add-on.