Optional Essay - expected promotion?

I wouldn't mention it, however, if you were to get the promotion after you submitted your applications, definitely contact the schools and let them know (and be ready to send along an updated resume).

I just think it's too "...and?" for an optional essay. You may be able to find a way to sneak it into interviews.

Another thought to consider is if you have your current supervisor writing your recommendations it may make sense for him to mention this or how close you are to being promoted. Recommenders can do a lot to reinforce performance sentiments and this would be a great examples if you are unable to secure a formal promotion in time for application deadlines

I agree with the advice that you should 1) have your current boss mention the expected promotion in his/her recommendation letter. It will mean something coming from him/her, it will not mean anything coming from you since it is not "real" yet, and 2) go ahead and ask for it early so it CAN be real and your resume can be updated as such. For you to talk about an expected promotion in the additional info field might seem immature to the AdCom.

I disagree with the comment that said you should contact the AdCom after you submit if/when you actually get the promotion. That's a big no-no in my book. Think of what would happen if all of the thousands of applicants in each round contacted the AdCom with updates like that post-deadline? They have deadlines for that very reason -- so they can evaluate candidates against each other within the same period of time. If you get an interview, then yes, definitely mention any new updates when you're speaking to someone in person. Same goes for if you get waitlisted--some schools welcome updates at that point.

Hope that helps! Best of luck in making the promotion official sooner rather than later!