Give my AdvancedReports module I try. It will allow you to build a report with any DB query which could be a UNION over multiple queries on multiple tables or even a storec proc on the database. mjvn.com/index.php/sugarcrm-tools/
It's a work in progress, but it can definitely do what you want. If you can write a DB query that returns the data, it can turn it into a pretty(ish) report for you.
Marnus van Niekerk
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