An orbeon integration with Activiti is possible. Look at this post here: http://forums.activiti.org/content/orbeon-xform-integration-activiti-workflow
It is right that Activiti only supports JSON. But with a custom formHandler, you could receive the XML posted by orbeon, parse the process variables, start a process inatance and write the updated variables back to the Activiti database. I just did a prototypical proof of concept about this. In this servlet, you can use the Java APIs of Activiti.
You could realize it like this:
1. Display an orbeon form which has fields for the initial process values
2. Submit those values to a custom servlet
3. The servlet parses the orbeon xml form data and extracts the typed in values
4. Use the Activiti Java API wihtin your servlet to start a process instance with the values extracted from the orboen xml
5. If the instance was started correctly, send a HTTPStatusCode 200 back to orbeon.
6. Close the orbeon form.
Best regards
Ben