All I can do is speak from past experience. I can suggest with some ideas, and you can decide for yourself if you think something might work for you.
I am of the same mind as you are - get away from the hard core drugs.
I have had allopathic vets in the past who find that the use of prescription drugs on small animals can be a lot more harsh with more severe side effects on them than on humans. In order to circumvent the problems, they have expanded their knowledge into alternative medicines and they have had good results from doing this.
The best part of finding an allopathic vet that has taken up the homeopathics as well, is that it gives you the best of both worlds.
Maybe what you might want to do is find a vet that practises alternative medicines. If this is not an option, there are some decent web sites that the majority of us use to educate ourselves in the art of natural feeding.
If I were you, I would be contemplating very seriously feeding raw. Either making it yourself or buying it. One of the problems with IBD is that we really don't know what has caused it. Whether it's an allergic reaction to something in the manufactured food, or stress over time. And by the looks of it, checking it out with the vet, hasn't produced anything medical to cause your problem. So it leaves either a stress problem or a food problem.
Something I would be giving the cat to try to cool the digestive tract would be Aloe Vera Juice - by Lily of the Desert - and make sure it is the pure juice with no preservatives added marked right across the front of the jar. We have found that it usually comes in a glass jar, and you get it at a health food store. You could start with 5 to 7 ml over the course of the day, doing one or two ml at a time.
If you're going to use Slippery Elm, I would also mix it with the Aloe Vera Juice. I've used the Slippery Elm in the past and had some results from it.
I have since gone to products called White Clay and Grey Clay. These products are a powder in a box and are by a company called A.Vogel. I mix the clays together 50 / 50 ( either a tsp or a tblsp of each ) and then take 1/4 tsp of the mixture and about 1 1/2 cc of Aloe Vera Juice or distilled water - if out of A V Juice - and mix it into a slurry or very thin runny paste so that it can be administered with either a syringe or an eye dropper. I give it twice a day and usually half an hour before a meal if possible. I mix it fresh each time. The neat thing is that it basically replicates Kaopectate.
If there is anything further I can help with, let me know. I have included 3 web sites for you to check out, that a lot of us refer to from time to time. Hopefully someone else will add some other suggestions to help you out.