I'm not in so into the UIs and the rest server but I'm pretty sure that the rest server is located here:
https://github.com/droolsjbpm/jbpm/tree/master/jbpm-gwt
More concrete the project: jbpm-gwt-console-server expose the rest endpoints to interact.
The logic of that services is coded inside jbpm-gwt-core project.
You should be able to fork that and create the appropriate tests to test your proposed functionality.
If you clone all the project you will be able to change the UIs and the server to test your functionality, but Unit Tests are always reuqired (Junit)
Cheers