In Jenkins labels might have configurations via label properties: https://github.com/jenkinsci/jenkins/blob/bfdd49537cf84f94a17559f2bd6c105d2b5e7c3e/core/src/main/java/hudson/model/labels/LabelAtomProperty.java. It is not the most common functionality, but is is used in two plugins inside the update center: https://jenkins.io/doc/developer/extensions/jenkins-core/#labelatomproperty 

Configuration management code is here: https://github.com/jenkinsci/jenkins/blob/811583eb6007ffa22f81c312d983816e5f36a670/core/src/main/java/hudson/model/labels/LabelAtom.java 

The code is a manual implementation, and JCasC obviously does not support it.

In order to support the logic, labels need to be refactored (e.g. to Global configuration category or so) OR there should be a JCasC plugin patch for supporting label management there