Cats can be trained to sit on and use a human toilet seat. A space toilet with a vacuum would be more challenging but I bet a dedicated animal behaviorist with enough time and training could get a cat used to using a space toilet.
It may just be easier to design a zero G litter box. A screen bottom with a coarse granular material and sufficient airflow through the bed could work. No idea how a cat would dig in zero G though.
Sturdy screening to give claws a grip would probably serve as the equivalent of human handholds just generally. I don't know how comfortable or feasible walking on it would be, but I bet a cat could learn pretty quick to catch on to it and then push off along a desired vector, and they're already better at gyroscopic pointing than we are.
