Earlier this week, I booked some last minute 2 MileSAAver award seats. As expected for a Gold member, I was not able to select MCE seats at the time of redemption. Last night, at around T-48 hours, I happened to go into the AA iPhone app to check on my reservation and tapped the Change Seats button. To my surprise, the seat map showed 2 open MCE seats which I could select. Before going through with the change, I went to aa.com and when I tried to change seats, those 2 MCE seats showed as grayed out and blocked. So I went back to the app and switched myself to those seats and was not charged. The change was confirmed and appeared in My Reservations. Not sure if this is a one time glitch or some bug in the app that allows Gold selection of MCE extra seats on award tickets prior to 24 hours.

At T-24 hours, the rest of the MCE opened up for me and there were plenty available.

Edited: Forgot to add that I was also able to select 2 MCE seats on my return flight through the app, 8 days before the departure date.