You can use a table calculation as a filter. The reason this works is that table calculations are performed after the data has returned to the Tableau data engine.
I've attached a workbook with some explanation that demonstrates this.
Table Calc Filter.twbx.zip 428.0 KB
Another solution is to duplicate the data source and use the duplicate as the unfilterable aggregate. It all depends on how your data is structured as to the best solution. It sounds like this will work for you if I'm interpreting your question correctly.