I am on AA 721 LGA to DFW on the 5th and just got my upgrade. While going in and selecting my seat, I noticed a new option "Entree Reservation" was on the screen. When I selected it I was able to choose the meal for this portion of the flight in advance. I have not seen that option before.
I am assuming that this is a new feature which I like. Now I don't have to worry about FEBO when upgraded more than 24 hours in advance.
Your assumptions are correct. It is a nice little perk. Started back around the holidays I do believe.