Worked for me nicely this week
I was a little skeptical, but decided to take a last minute quick trip to MYR from ATL. Bought a main cabin ticket using 19K miles about 6 hours before the flight. Apparently the only Diamond on the flight as I went right to the top of the 26 person First Upgrade list per the app. About an hour before pushback, I got the one and only seat available in first.