DB API: How to add the result of an embedded query and a retrieved value
This was indeed a database question; a fix for the "unknown column" problem can be found at https://stackoverflow.com/questions/14879342/can-i-do-math-on-the-result-of-an-embedded-query-in-the-parent-query (short answer: the computing of count_task_views needs to be done in a lower-level SELECT).  Back to Drupal: I have my query working as pure SQL handled through db_query; I should probably figure out how to do it via the API, but that's going to wait for awhile...