We've discussed it here as well. The G Deck location is cited on the Eaton & Haas plans, but I wonder about it's veracity. The problem is that their location puts it right across the passageway to the 3rd Class Galley which would raise some serious concerns for health and sanitation.
I've seen some specualtion for the boat deck too, (Right here on ET.) with the idea being that a deck chair storage was converted over for the perpose. This would make more sense IMO and would be fairly easy to do. Unfortunately, if any documentation was written up to the effect, it's either lost in an archive somewhere or is no longer extant.