I hope Coco feels better soon. Just a thought - I'd just been reading about Precious Cat Senior litter on the Dr. Elsey (of Cat Attract fame) web site. I wonder if this would be worth a shot - below is a quote form the web site. I'm keen on his litters, because Cat Attract worked wonders with my boy.
Precious Cat Senior litter addresses the medical and urinary tract problems that this specific group of cats experience with an amorphous silica gel litter infused with hydrolyzed herbs. This litter will help prevent kidney failure and urinary infections that can cause non-use of the litter box. Precious Cat Senior litter has a small particle size that cats prefer. Senior litter absorbs urine on contact and traps it inside the crystal to prevent bacterial growth, thus helping to prevent urinary tract infections. Its small particle size and dust fines also coat and dehydrate cat feces to reduce odor and prevent bacterial growth of E-Coli. E-Coli bacteria grow on feces and along with reduced natural body defenses can lead to kidney failure, urinary and uterine infections. Precious Cat Senior has no organic material such as wheat, pine, corn and paper that can support bacteria and fungal growth, there by helping to prevent urinary infections