For cats who won't eat food that smells of ACV I have heard of some alternatives. One is to dilute the vinegar with a little strong chicken broth, which most cats will eat eagerly. Another is to put a few drops of diluted ACV on the scruff of the neck and massage it in so it gets absorbed into the body.
On another point, since cystitis may come from toxins in the body an easy way to detox your cat and possibly prevent recurring cystitis attacks is with Chlorella, a nutrient-packed natural food from the sea.
I grind up the little tablets into a powder and put a pinch in my cats' wet food once a day. My 13 year old is healthy, her eyes are clear and she plays... more than before the chlorella.