>> Is there any way to save the bpmn2 process, from my own save button? <<
Yes, but it would involve some trickery:
1) First you have to get the JSON representation of the process via JavaScript:
window.frames[X].ORYX.EDITOR.getSerializedJSON()
where X is the number of the frame in your doc
2) Pass the JSON to designer to get it to convert it to BPMN2 (post):
localhost:8080/designer/uuidRepository?profile=jbpm&action=toXML&data=<YOUR JSON HERE>&pp=
3) Use the Guvnor rest api to create a new asset with its source being the BPMN2 you got back from Designer:
see this class as example on that...https://github.com/tsurdilo/process-designer/blob/master/src/main/java/com/intalio/web/server/TaskFormsServlet.java
Hope this helps ;)