RE: Activity Reporting with Email Message/Memo has HTML
As far as I am aware, you cannot add scripting or other handling instructions to reports. If you could run these as saved searches, you might have more options but as you say they may time out. Have you considered an Excel web query with a macro to reformat the data? The solution really depends upon how big the data set is that you are mining. Sometimes you can script a report that gets successive short result sets, builds an array and sorts via code if absolutely necessary.
On the other hand, do you really need the messages themselves in the report?
Mark