But against a straight punch down the center, he would definitely be parrying at the wrist area.
Certainly it can happen - parrying at the wrist area - and can be pulled off safely. However, I think that as KPM was saying, that wouldn't be the timing and distance management I would be looking for.
Assuming I am in Man/Wu, I ideally want to be moving in and want to deal with any incoming straight hand shot from the opponent when their strike has moved inside my Man Sau. I think a Pak Sau at their wrist would be engaging a little outside of the range I'd want to be in. Basically, I want the exchange to be closer.