I have tried the ReporItems reference as you mention. However, the error I get on that is that ReportItems can only reference items in the Report Header or Report Footer, not a Group Header or Footer.
I have tried referencing the scope value as the group name of the higher level group. That doesn't seem to help. I get the same error.
I have considered modifying my stored procedure to supply the values, however, I am having to use the OPENQUERY method join data from an Oracle Server in another domain to data from one my my SQL Servers in my departmental domain. I would have to use a number of correlated subqueries that would be demanding on both servers everytime it ran. I am hoping to avoid that option if at all possible.
Running Sum is an option that I have considered, but was hoping to handle it with a reference to a higher level group if possible.