Adding a custom Sort field in Drupal
Hi Rich who isn't the normal Drupal developer and was left to learn on his own. Welcome to Drupal.

I've encountered scenarios in which I had an exposed sort with a dropdown list and I needed to remove all the options that didn't fit my pattern. I used a hook_form_alter() that modified the '#options list for the element to unset() the items I didn't want to allow the user to select. However, this method doesn't prevent those items from showing when -All- is selected, so that may not work if that's an option for your users.

The way you'd add a new field is to create a custom field handler but it sounds like what you want might instead be a custom sort handler or a custom filter handler. The three are similar but not identical in their implementation.