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 ...
I have simliar requirements. I would like project managers to be able to view their own projects by default, but to be able to remove the filter if necessary (eg. if they wish to look at projects for another project manager). Row level security works to filter to their own projects, but you can't then remove the filter.
