Another option would be to creat the new dimension based on a calcuated field. Similar to the SQL query that you outlined above, but there might be similar limitations to the ones experienced with the query, i.e. not truly dynamic.
Qucik question, does any relationship exist in the data tables that could be used to distquish when they are in or out of the county? If we could find a relationship in the data, these could be used to build the calculations for creating the in county/out of county.
Another option would be to use sets, creating an "in-county" set and "out-of-county" set that could be used for filtering in the workbook.
I would also love to see a workbook attached! It helps our Community users assist quickly and easily.
Hope this helps!