They could always re-crop as best suits the 16:9 and then use digital processing to remove any boom mics or other unintentionals that happen to be in the frame.
Yes, that makes it an even more mammoth project. But as technologies advance this kind of thing will only become easier and easier to do.
I understand the arguments about re-cropping screwing up the framing. Certainly I wouldn't want them to recrop to a smaller version of the original 4:3 shots. But, if they used the wider pre-cropped stock, and kept the original 4:3 intended shot at center frame while adding additional information around the edges, I think that could work really well.