I answered your question of a week ago. I take it you did not try the slippery elm bark. A friend has a cat with IBD. She has been using slippery elm bark for a few months now with good results. He is at least 75% improved. He does have soft stool sometimes, but no more diarrhea.
She likes it so much she just asked me to get her some more before she runs out. I shop at a place that sells bulk herbs so I got what I thought would be a 6 month supply - $1.86!
She makes up the solution, stores it in the fridge, and adds a teaspoon to each meal.
http://www.littlebigcat.com/?action=library&act=show&item=slipperyelm
Some other substances that may help your cat are the amino acid L-Glutamine and Larix occidentalis arabinogalactan.
I mentioned the L-Glutamine in another reply. This is used in humans, cats and dogs to help heal the intestinal lining. It is not a drug and you can buy the capsules and sometimes the powder (cheaper) in health food stores. Please google for the correct dose in cats.
Larix occidentalis is the a species of Larch tree. The arabinogalacan is a substance obtained from the bark of this tree. Amongst other issues, it is used in intestinal disorders including IBD in humans and other animals. For a cat in my family we used a human dietary supplement containing this and it worked quite well. I will try to find the name of it and update.
I suggest you do some internet research into larch arabinogalactans for cats with IBD.
I will try to find some sites for you myself and update.
Probiotics are a good idea. I have a caveat concerning them though and that is that many probiotic products are no good. The reasons are that some are badly formulated using organisms that kill or eat each other leaving only the dominant strains listed on the label, that the product is not storage temp. stable and the organisms are dead. I can't give a comprehensive list of all the good products but one is Jarrow EPS.
With diet, I would try once more. Give your cat just one food, an unfamiliar protien source like duck or rabbit only and see what happens. At this point, after what you have tried, I doubt it is a food sensitivity though.
Update: Found this holistic vet's site. He also sells some products containing substances mentioned above.
http://www.homevet.com/
Larch info, scroll about 40% down this page:
http://www.vrp.com/newsletter.aspx?newsdate=8-1-2005