insert in a matching category globally?

I’m using template theme file code snippets.
Is there a way to cause the snippet to show on any individual Post that is a member of a specific category?
Right now, using WP-Insert, I can make the snippet appear in an archive index page of a specific category, or a specific Page, but what I want to do is allow a sponsor to have an ad appear on all Posts of a specific category, but no other categories. I don’t see a way to do that at the post level.

http://wordpress.org/extend/plugins/wp-insert/