You might also try posting flyers at really good veterinary offices in the hopes of attracting a sincere, committed person to give her a forever home, preferably someone the vet's office knows. Having done a lot of adoptions, I found that I had to screen very thoroughly to avoid placing an innocent life in some wacko's hands. The wacko's often seem sane at first encounter - they know how to play the game.
As to the litter box avoidance, your flyer should probably say that there are litter box challenges at the moment. Animal behaviorist Warren Eckstein (see thepetshow.com) provides guidance on how to get kitty to faithfully use a litter box (all my rescues have readily used litter boxes so kitties are usually pretty good about this). He can be reached for free on air questions when on WOR Radio Saturday afternoons. If the litter box situation is not resolved, the kitty may be best suited for a warm comfy barn life with all other perks (food, water, vet care...), spayed of course. Friends of Animals offers reduced cost spay/neuter certificates.
Sending good wishes for a wonderful home for her.
JJ