Just checked with my SE.
She says can drop the keys before 7am at the guard house then pick it up after 3pm on Sat.

Yes, I doing that as I am driving an OPC. You can do that... the servicing centre at BT is opened from 6am onwards, the security officer will tell you where to park, lock the door, drop the key in the envelope provided (just outside the servicing office), write your particulars. The service man will call you later in the morning.
This is one consideration for me before buying a nissan.
Think of it, quite convenience for OPC cars....