A print-out was fine with SQ when I flew before getting my A3 card - though this was on a revenue ticket and they were putting the number into the FQTV field on the booking, which would give verification of *G status.

In your case, while SQ will not put a non-SQ number in the FQTS field, they may be able to put your A3 number in FQTV while your SQ number is in FQTR (assuming a SQ redemption booking). Or they may be OK with the printout.