I have heard that a completely raw diet has cured some cat with servere and chronic diarrhoea. Whether that works for all cats or not I don't know, but some of the stories I read were dramatic.

I am sure there used to be a Yahoo group for IBS but I can not find it now. There is a Holisticat forum, you have to join to see the posts, but that looks to have recipe sections for a wide variety of problems Holisticat Community :: Index

They have a few recipes on their main site. This one is supposed to reproduce the Hills Hypoallergenic diet

The Original Holisticat (TM) Homemade Rx Diets

Have you had her allergy tested to see what foods she is allergic to?