Hi Lisa,
An interesting idea, thanks. Still it gets more report variables involved -- I'm not exactly comfortable with that.
So far I decided on excluding all total and group calculations from the report and moving them to data prep stage.
On that stage I create several totals cursors -- one per each report group and one for the whole report -- and link them (SET RELATION TO) to the report's main cursor.
Having all total fields already calculated I'm able to format them in the report using an external function.