Here is a proposal to add a principal-transformer attribute to aggregate-realm

https://github.com/wildfly/wildfly-proposals/pull/209

This is being handled under the following Jira issues: -

https://issues.jboss.org/browse/ELY-1829
https://issues.jboss.org/browse/WFCORE-4496
https://issues.jboss.org/browse/WFLY-12202

Regards,
Ashley Abdel-Sayed