I used to hack the Activities BAO to change the search results to make the subject line something more useful; i.e. extract other data from particular activity types and auto-create a summary in place of the actual subject.
I'm doing an upgrade and realised what a royal maintainance PITA this has become :-)
I thought I could implement hook_civicrm_searchColumns but I've tried from a Drupal module and also from an extension created for the job but it does not appear to be being called for the tab selector? Is there another way? I've tried this on 4.6 and 4.7.