Today this isn't practical. The problem is you can't combine PA scores. For instance, if you have an indicator counting distinct anything or averaging something you can't simply add them or average them.
The alternative I have seen is to create an additional breakdown that combines the elements you normally want to combine. This is very similar to a category/sub-category fieldset. You rarely need to select multiple sub-categories because you can just select the category. This isn't perfect but seems to meet most of the use-cases I have seen. For assignment groups, I leverage the parent group with a hierarchy. For non-hierarchal data, I sometimes use a BR to set an additional choice list on the base object to group the individual fields into the roll-up breakdown.
