Hi L.Elias.
What if the audit trail isn't a reporting tool? I recommend using the process engine to 'date stamp' the events you're interested in. Below is an example of where we date stamp when the investments move into stages and each night 'count up' business days in that stage (we stamp other events too). We use attributes to 'capture' this data - therefore it is available in Clarity on the object, easily available for portlets, reports and - once we have Jaspersoft - it's a checkbox away from being in the datamart.
I think it's better to put the effort into getting the information into the right spot vs. building reports against something that's not built to report from (and - is not supported to report from).
FYI - We built all this first out of the Audit trail (built a nightly job to 'datamart out' the dates and days), but then the next layer of this onion was that, in order to get at this data everything was custom reporting. Then CA changed the audit trail... shame on me - again, this was not/is not a supported integration/reporting point.
The above approach is so much better since the data is now in a fully supported spot in the datamodel that all user can get at (above is the project list!) and do with it what they wish.