Being restricted to just using username () in measures (or AllowedRowsExpression) means that you cannot use the context of the current user in slicers or page level or report level filters. Just ...
Also, the comments below do not address what Brian is trying to do. Power BI RLS is post query, not at query time. Also the solution offered by anonymous to "=Sql.Database("(localdb)\v11.0","",[Query="SELECT SYSTEM_USER"]) should do it" will not work as most queries, particularly if going through a gateway will have the service account context not the user running the report.
