Hi MC,
I personally don't have any experience with it, but this is what I found on IBDKitties.net (
http://ibdkitties.net/tipsandtricks.html). By the way, she also suggests canned pumpkin to help with constipation, but I think you've tried that and SK wouldn't eat it, is that right?
Tip #1: When giving slippery elm bark for nausea, take a tablespoon of hot water and mix it with about 1/2 capsule of SEB powder. As it starts to cool, it will thicken. Sometimes you can add more powder and sometimes more water to get it to a thin enough consistency that you can fill a syringe. The easiest syringe to use is a 3ml, and fill completely. Then insert the syringe in the back corner of your cat's mouth and let them drink it down. Just be sure to make it as thick as possible as it helps coat the lining of the GI tract, similar to Pepto Bismol. (NEVER use Pepto Bismol). They call it a "liquid band aid". Slippery elm should be given at least two hours away from any other medications or supplements. 30 minutes isn't long enough for the mucilagenous coating in the digestive tract to dilute enough to let absorption take place, mostly in the intestines. Many holistic vets believe that when given with a meal, it can inhibit the absorption of nutrients from the food.
And here's a link to an article by Dr. Jean Hofve on littlebigcat:
http://www.littlebigcat.com/health/slippery-elm/. You might want to do a search on her site for slippery elm, because she may have more info elsewhere, but this looks like the main one.
I hope this helps!