If you want to write your own authentication plugin, please refer to this link. We have implemented OpenSSO on On-Premise for one of our clients. I don't have the complete details on how to do it. If you want, i can get the details.
Regarding the integrating-applications documentation, which you are reffering, is for other applications to interact with applications created with OrangeScape. All OrangeScape application's objects are exposed as REST as well as SOAP webservices.