Thanks, you've hit the nail on the head......these controllers connect the FB pin internally to an internal overvoltage comparator......so its not actually even possible to add a comparator and do your own Overvoltage level setting. [apart from that is, of course, on the LT1248, which is the only single PFC controller on the entire market which allows to do what you kindly describe (add a comparator)].
..I suppose its my fault for being such a Luddite and not using a microcontroller based PFC controller.