I don't blame you for being cautious, but FWIW I did manage to split the PNR on an international award ticket involving SQ and other carriers a few years ago. But it took some doing, including finding the right UA phone agent to handle this, and then triple-checking to make sure it worked out right. If you want to pursue this, you could try the UA international awards desk at 888-674-4680, some of whose agents are pretty savvy. But I wouldn't blame you if you decided to leave well enough alone.