Just a give some data point for hotel in Japan and in Malaysia:

Hotel Nikko Narita
Requested for early check in upon arrival at 7am (regular check in time was at 2pm) with no prior communication made.
I am required to pay 50% of nightly rate.
Room key was immediately given to me and I was able to get inside the room.
Bear in mind this is on Monday morning where I assumed the weekend crowd have left on Sunday there have some room ready.
Not sure if it is possible on peak dates.

Airport hotel within Osaka Itami Airport (I think its name is literally Airport Hotel)
Regular Check in time at 3pm and I arrived at 11am, again no prior communication was made.
I am required to pay the hourly rate between 11am to 2pm in order to check in early.
Key was given immediately after payment done.

Sama-Sama Hotel located beside KUL (the one located on landside and not Express which is airside of KUL)
Arrived around 12pm, regular check in time is at 2pm
Requested to front desk if its possible to check in at 12pm.
Request was granted and I am not required to pay any extra cost for it.
Its on regular Wednesday so things maybe different on peak period or holiday season.

Hope this data point helps