I have also, in the past, written posts in emails to myself or notepad for the purpose of posting them later, by literal copy and paste. There is nothing in the rules that prevents this, and I don't think there should be.
The issue I could see with the QT is it's something given by the mod that the mod can see; there is a slight gray area with "mod communication". For example, my first post that QT was something like "I'm thinking of doing something crazy this game". Was it directed at the mod, or to the general aether? It could have easily been "Hey, Yuma, I'm thinking of doing something crazy this game".
You could have the rule be something like, you can't post anything from any medium of open communication (including QTs), but you can compile messages ahead of time just for yourself and copy them in (e.g., notepad, email to self).