You can check which pages are active in the opened XML editors, modify them, or add listeners to notify custom views that can provide additional context information. For instance, it is possible to access the document model from the Author page, directly from your plugin, using most of the Authoring API.
The oXygen SDK contains the oxygen-sample-eclipse-plugin module (a complete sample Eclipse plugin that extends the oXygen XML Editor Plugin for Eclipse).
To build and package the Eclipse plugin, follow the steps described in the README.html file from the project directory.
The development of extensions to access oXygen XML Eclipse plug-in features from your own Eclipse plug-ins using the SDK is free of charge, subject to registration and in accordance with the SDK agreement. Testing the extensions during development phase can be done using a licensed oXygen XML Eclipse plugin.
There is no restriction on how you can license your own Eclipse plugins (that use the extensions) as long us your Eclipse plugin works together with a licensed copy of oXygen XML Eclipse plugin. You can use your own licensing terms (including commercial), the only exception is that you can't use GPL, as oXygen XML is not GPL.