I think the "A" could work just as well as an X for people who don't know which function is dominant. But in my opinion, it's saying exactly the same thing: "I know I prefer this Introverted function, and this Extraverted function, but I don't know which function is my auxiliary, and which one is my dominant." Thus, I question the value of introducing a new way of representing the same idea (it may create confusion), although I do not see real harm in doing so.
Granted, there may be genuine confusion here for many people, especially if the development of the two is close together. But I do still think everyone has one dominant function that they prefer, if only infinitesimally. I will admit that if they're close enough in development, it may be near impossible to tell which.