Is there an extension for adding a custom field type calculated field?
Paul, very interested in this idea as well. Don't have a great answer, but here's an idea - since you're on Drupal, I wonder if this would work - using Views, Views Calc and Views Aggregator Plus (perhaps along with webform-civicrm) to do the calculation - add the survey answers to get a score - then saving that score back into Civi in a custom field?