There is already an ability to create a compound query by manually editing QML files and you can assign a type to these compound queries of union or union all. Howerver, if any of the sub queries being unioned return no data the report will fail. After speaking to a support analyst it seems the compound query types union and union all are not supported (though they are available).
This could realistically be a resolved in a phased approach
1) Enable/fix the union and union all compound query types that seem to already exist so they funciton properly if any of the sub queries return no data. The user will need to manually build the QML.
2) Introduce the compound query capabilities into the Report Manager itself.
Being able to create compound queries will be a huge benefit for querying/displaying system data.