I was kind of thinking the same thing about a small warm water enema, but it would be so much trickier with a J pouch because you've got no colon and who knows how many times a day you'd have to do it to get any kind of substantial relief for the anal area. And then of course your fluid balance and nutrient balance could get messed up because of the lack of a colon. As you mention your diet is probably constantly undergoing tweaking and diet stuff is so individual.
I don't know if you can tolerate soluble fibers or gums, which unlike insoluble fiber like grains and seeds, do help absorb those digestive acids. Pure soluble fiber in the form of an acacia powder maybe? I use acacia powder as I too don't tolerate the cholestyramine well. I'm also wondering about something like slippery elm for you if you haven't ever tried that. Really innocuous and can help soothe digestive tissues from exposure to those acids.
You have been through so much surgically already and must feel so frustrated with all you've been through to be dealing with this now. But like owmybum said above, I also want to welcome you aboard and hope that you feel some relief knowing that we do get it when it comes to what it's like to deal with chronic anal pain.