Yeah, this was missing in the API. I opened a JIRA in https://issues.jboss.org/browse/ROASTER-68 to fix this.
The fix is already in the master branch. Could you give it a try with the latest SNAPSHOT and let me know how it goes?
Thanks so much George. I can confirm that setStatic works on nexted classes on 2.13.2-SNAPSHOT
I just released Roaster 2.13.2.Final. Should be in Maven central in a few hours.