Sounds like you could create a calculated field with WINDOW_SUM(SUM([Sales])).
Then you would just need to make sure you include the [Region] dimension in your view in the Rows or Columns shelf.
The calculated field will generate a Table Calculation, for which you can control the field(s) used to group the "windows."
I've just faced the same problem - I need to aggregate some measures across the dimension that is not used in the view.
When I used QlikView earlier there was a powerfull function AGGR(avg(measure), dimention1, dimention2..), but I couldn't find anything simmilar in the Tableau.
Did you solve this problem already?
Did you try custom sql?
Actually I have tried the function you mentioned for Qlikview.But in Tableau we don't have any function similar to that.But Tableau provide a feature called table calculations using that you can aggregate measures used in view.But for that dimensions should be there in the view.