Although it does not pertain to the OP's exact predicament / travel itinerary, here is another experience with *A and OW interlining:

SQ did check through my bags onto subsequent QR flights (SIN-SGN, then SGN-DOH-FRA) last month without any problems. I am doing the return run in a few hours and will report if QR are willing to do the opposite in FRA.

Update: So, QR at FRA would have checked my bags through all the way to SIN with the last leg on SQ, only that my bags were overweight, and they would have had to collect payment for the excess baggage. Hence I told them to only check the bags through to SGN, where I will try my best into sweet-talking the local SQ staff to waive the additional fee (56kg instead of 50kg) for the last leg SGN-SIN.