This would almost certainly get me fired, but I would simply refuse the request. If I truly don't believe anyone on my team has earned themselves a spot on a PIP I wouldn't put anyone on one.

I have to expect the response would be either (a) threatening to fire me or (b) threatening to force my boss to make the decision with less knowledge of each of my reports' performance. In either case, though, those aren't my decisions to make and I can only take control of whether or not I'm willing to PIP reports that I don't think deserve it.