I'm not sure there's an answer to this -- but I'm interested to follow this thread to see if someone comes up with a solution.
Something you could do, but I'm not sure how scalable or long-term manageable it is, would be to micro-define your content categories. For instance, let's say you are promoting Content A --
Email 1 doesn't reference Content A, so you can leave your Categories as ALL.
Email 2 references Content A. In ContentAI you mark Content A as "Category 1" and all of the other content "Category 2". Then in your email when you insert ContentAI you select a category of "Category 2"
As I mentioned, this won't solve your problem long term, but if you need to get an email out that will occasionally by included in ContentAI, then this would be a feasible hack.