I have sold ozone systems as far back as the eighties. Del sent some engineers to my showroom floor to take measurements and size up the tubs for systems which used the the circ pump and some of the built-in plumbing. Watkins did not support that, and generally asked that we dealers not encourage ozone sales. I think they were concerned over the fact that some of our customers would go completely chlorine free and get a rash. Now, they combine ozone with silver ions AND MPS oxidizer, backed up with occasional dosing of chlorine.
Also, they redesigned the tubs with a "contact chamber" which extends the amount of time the ozone is in contact with the water, greatly reducing off-gassing and making the systems actually work!
I know HotSpring announced a new ACE system which retrofits older HotSpring tubs - I just can't recall how far back it goes. I can find out on Monday if anyone wants to know, but I do know that it has a control head of it's own instead of integrating with the IQ-2020 box. I know that you must have the 'pass through' fitting in the filter box, and I don't think it will work with the IQ-2000 system.
HTH