SQ have always told me that the operating carrier pays regardless of what it says on the *G card. Back in the day when SQ was in T3 at LHR I knew all the lounge staff well as an LPPS customer who was often on SQ but when I was on AC, SK or others the joke was always who was paying for my lounge access today, SQ or another. So it looks like the operating carrier pays and as they are deriving the revenue from the booking that is the right way for things to be.