Ok to bring this to a close:
Arrived adjust part midnight the other day and was directed into the F lounge.
Asked to use a slumber room, and they gave me a key for it.
Used it till 630am the next morning and proceeded to my domestic flight connection.
So, it is possible to use the slumber rooms overnight in the MH F lounge.
Now, will I do it again? I am not so sure. What I found was the walls were rather thin, and I could hear the snoring from a passenger in a opposite room (thankfully they left around 2am)! Also it did get someone noisy from time to time, with the cleaning staff outside doing their own thing. Still it is free though... Hard to bear that price...