We're happy to announce that we've just released the final piece of app notifications. You can now send app notifications directly from an OAuth REST end point. The REST end point allows you to pass in up to one thousand (1000) recipients at a time making it ideal for mass messaging communications to users of your app. This method also allows you to send notifications on a true server-to-server basis which is not initiated by a user action.
The general notifications page can be found in the developer Wiki. Documentation for the REST end point can be found here.