We already support custom meta data, and according to this thread on stackoverflow you can use the standard name and content meta data attributes, so you can use open graph protocol in mojoPortal today.
According to the documentation
It is currently designed for Web pages representing profiles of real-world things — things like movies, sports teams, celebrities, and restaurants.
so it isn't the kind of thing you would put on all pages of your site, maybe on your home page or pages about specific products would make sense.
Hope that helps,