We have been lucky enough to pre cruise at the GO Lounge 2 times. The first time we were in a reserve class mini so we wanted the Sanctuary. The concierge at the Lounge (Anthony) called over to the concierge on board the ship to take the reservations. Being a little cynical, we still went up to the Sanctuary as soon as we boarded to double check the reservation. Manager said he know nothing about it, however, luckily he was able to fit us in for the week. I believe that everything may depend on how quickly the concierge on board takes care of the requests.
Our second time we were in the GO Lounge we were in a full suite and didn't need the Sanctuary.
The Lounge is a very nice pre boarding area. Just double check the reservations when you get on board.