Have we considered that maybe the code words aren't static, and instead change depending on the context of the situation? Perhaps the code words have a general meaning but can but can be used differently depending on the situation. This allows them to continue doing it without anyone ever being able to pin a single meaning to one code. For instance, maybe if we looked into events going on in the background around the time this email was sent, and do that for every email including Asparagus, we can narrow down how they're using it based on repeating patterns in their behavior outside of email.

I'm just speculating, but something that supports that idea is how quickly we found the handkerchief code and applied it to the emails. It just seams too easy, especially if other sub cultures use the same code for their own legal sex. Why would they abide by that code if it's well known, or easily verifiable? Maybe because although they use the same code word, the meaning of that word changes based on the receivers knowledge of events going on in the background.

If a foreign diplomat wanted a play date with podesta and some kids to fuck together, but that diplomat really dislikes pizza as a food, would it be a good idea to use the static code word of Pizza for someone who hates that type of food?