Why not? It's as good a word as any, and if you make it a word that's naturally associated with him in some way, it makes the code easier to break. If you and Astrid set up a code that always identified me as "Red", it'd be much less secure than if you decided to dub me "Footstool".

[Not that she particularly wants to be dubbed "Footstool", mind, but hey. An example's an example.]

And it doesn't matter what's in the notes. What matters is they're yours. Which means you also have a right to have people not read them.