How about using a table calc to do that, and then put the new field on the Level of Detail shelf, so that it appears in the tooltip? I used the superstore database and here is an example.
Hope this helps!
Grand Total table calc.twbx.zip 185.9 KB
I am using public, is there any way I can get you to publish this to public and provide a link? This was the approach I figured I would need to take and so am looking forward to seeing your example.