Change Item Drilldown Template by Product, or insert script

Hello.  We are using a website theme which works perfectly for all of our 7 product lines.  But I was wondering how one might modify an in-use product drilldown template to act differently depending on WHICH product line was being called.

I dug through the documentation and cannot determine how to pick a custom item drilldown template and apply it to one brand on one website. If I go to Website Content Manager and view categories, I can select custom  Category List Layouts, Item List Layouts, and Related Item List Layouts.  I don't have the option to pick an Item Drilldown template in these categories.