That may well work mate, though I personally have never tried it. I also can't claim to be nearly as knowledgeable as many others on this forum, so hopefully one of the cacti savants will chime in.
That being said, if you can graft the bottom half of a seedling and as long as there is an areole it will pup and grow, I don't see why this wouldn't work. You'd still have to remove the pereskiopsis though, which would leave a deep wound that may well become a very inviting inroad for infection. I'd be interested to see your results though, I know how it feels to have to split up a lovely multi headed loph and I'm not a fan of doing it. Good luck mate.