Yeah, you can shape (round over) the edges where the rope passes over the top of the spar with the chainsaw as you cut the notch, to give a really good bend radius. You can also cut a notch into the front side of the spar, opposite the knot, to reduce the chances of the rope riding up the spar, on smooth or flaky barked trees. Thick, corky barked ones grab the rope well.
It's a neat trick with only one caveat, really... if you're pulling the spar over, not just chunking, you need to have the rope guide notch pointing toward the lay of the fall, and perpendicular to the falling cut. Otherwise, the spar can tend to roll and twist off the stump, which is what you're trying to avoid in the first place.