I can't recall if it is in Part23 certification, but gear up landing is usually considered by the manufacturer. It is not hard to put skid rails on the bottom and older Pipers actually had a retracted gear position that left some wheel exposed in the event of gear failure. Like Rick says, you shut down and then feather. I think if my day comes to do this one however, I will make the approach a bit on the high side to give time to see if they feather first. That's where diverting to a longer runway may be helpful.