OK, made it work.
Since the Administration.Account already has a generalization, you can not set the AudittrailSuperClass. However, adding a reference set association to and copy and use the event MFs after changing the input object to Account did the trick.
I didn't try it myself, but can you try Administration.Account (that is a generalization of System.User).
It would have the same purpose, if you don't have logic in the App that create a System.User directly. And that would be possible to see in a XPath query since System.User Entity has Members activated:
Go MAke It