Just a data point for you guys:
I stayed at the Conrad Tokyo a couple of months ago with Diamond status. I arrived at around 7:00 AM and the hotel would not let me check in early even though my room was ready. They wanted to charge for an extra night which I declined. I did get upgraded and they gave me free breakfast for the day I was checking in since the lounge wasn't open for morning service. They finally let me check in at around 11:00 AM.