This is currently not possible since the expressions run off the compiled row and the aggregations happen in the database.
I think the simplest thing to do would be to port the expression column to the data source and then use a normal field column off that. Is that an option for you?
The Expression column(status) is computed from another report column(doc count) which is already an aggregation of count_unique.
The status column can also be done via conditional aggregation , but conditional aggregation only works for static reports and also, I don`t know if aggregation by count works on conditional aggregation type column.