Just a data point (and yes I rolled the dice) I checked in at 6.30h at the Bangkok Pullman G (Suite booked). Lounge receptionist warmly welcomed me back and checked me in without any problems (they also offered a discounted breakfast in the lounge which was perfect). I had requested early check in in advance but it certainly wasn't confirmed. Maybe the fact that I used the hotel transport from BKK helped.

The earliest I have ever checked in.