>> Attempting to do so at Comfort Hotel Osaka specifically.

You pay the double rate. Single rate at this hotel (Comfort Hotel Shinsaibashi - there is only 1 comfort hotel in Osaka) is often 1/2 the double rate, for the exact same room. At least I paid the single rate that was 50% off double.

Trying to have an additional guest without registering is considered a serious matter. If found out, you will be asked to

- pay the additional rate, or
- vacate the room and leave the premises immediately, or
- police notified

The lobby of the hotel is on the 2nd floor, accessed by 2 elevators or stairs. All guests must go through the lobby if using stairs.

For hotels in this category (business hotels), front desk people typically can recognize the guests after the 1st night. Also there is posted rule that non-registered guests are not allowed inside the guest room at any time. Visitors should meet staying guests in the lobby.

Even in 5 star hotels like the Peninsula Tokyo, there is a rule that non-registered guests must leave the room after 9 pm.

The only hotels you can try to do this are the large 4 and 5 star properties where the front desk cannot monitor it.