Jill, the Xanthan may be the way to go. Guar Gum can add a little bit of a flavor in our experience. Like Jeff said, they are both a little tricky to get the right consistency. We use it [Xanthan] to thicken fresh crushed strawberries as a topping for crepes at the brunch we cook at. It doesn't change the flavor of the strawberries and we don't have to add sugar or make a thin jelly.
If you have trouble finding it near you, let me know. The amish store just out of town sells it cheap and in bulk, I could mail you some.
Sorry you didn't get any responses to your original post ... very interesting problem! What are you using in place of the butter?