Unfortunately, you are doing chemisty experiments on your cats, since anything you put in the cartridge will leave a residue in the box, which will get on your cats paws, and from there into their mouths, since they lick their paws.
I have no idea what the basis is for your assertion that it is "completely safe for animals".
The Oxyclean has a high PH and can damage mucous membranes in the mouth and throat. The Scope is probably ok, but I cannot be sure of this, since they *are* going to swallow small amounts of it regularly.
It is probably better and safer to add this concoction at the *beginning* of a cycle occasionally, since then it will do it's cleaning, and be flushed out at the end of the cycle.
The box does not have to be sterile. Just running it on plain water 3 times a day makes it far cleaner than any box using traditional litter. Cats don't get sick from regular litter boxes, but they very well may get sick from home brew solutions that leave residue in the box.
I keep trying to make this point about the cartridge - over and over.
