Hi Stefan Heinrich,
At this time we are planning on making the Pipeline creation and editing work with Git based SCM systems and unfortunately there are no plans to make this work with Rational Team Concert SCM.
However, we do plan on allowing users to access the editor without having it attached to a version control repository.
JENKINS-43118 will allow you to copy a Jenkinsfile into the editor, use the editor to author your pipeline and copy a new Jenkinsfile out. From here you could commit the Jenkinsfile back to RTC version control.
If any community member wants to make the editor work directly with RTC version control, please get in touch with us on the Jenkins Developers mailing list.