[Xyberduke]

It is called transit rooms.
Not all hotel 81 offer transit rooms.
In GL, yes, but ironically, the bigger 81 allocates lesser rooms for transit or the possibility of no transit room when you arrive is higher.

For any nationality as long as you have a valid visa. If you have a white card, you need that as well. Visit pass, Student pass, work permit, everything are accepted.

Knock knock who is there happens frequently, so don't even think about bringing overstayers.

Can advance book transit rooms? I don't think so.

Just book for overnight stay for ~3x the 2-hours rate.
Very worth it and you get better room allocation, not one with moaning and wall banging next door.