For what it’s worth - to my understanding it all depends on the location of the 02 sensor in the exhaust system.
My previous bike was a Honda NC700. Cat removals has been discussed at one of the NC 700 forums and owners started removing cat convertors without having any problems. At some stage somebody mentioned that the reason for not having problems, is the specific location of the 02 sensor. So I decided to remove the cat on my Honda as well, and I never experienced any problems.
Unfortunately I can not remember at what spot the sensors are located at the Hondas - but I guess cat removals on these scooters should also not cause problems if the sensors are located at a specific position.
As far as these scooters are concerned - I guess unless somebody remove a cat, or somebody install an aftermarket muffler without a cat - nobody will know whether a modification like this will work.