If a pine tree is relatively healthy then the pitch will seal the top relatively quickly. If the tree does not have enough pitch to seal the top then it also does not have enough pitch to reject pine borers and will most likely be dead in a year anyway. Therefore trying to outthink Mother Nature by sticking a PVC hat on top of a topped tree is a waste of time IMO. Abut the only thing you can accomplish is extracting a few more dollars from the property owner's pocket.