A great thickener that I have been using is called "Thicknthin". It is pretty widely available and it only takes a tiny bit to thicken a large amount of liquid. It seems pretty pricey at first but it lasts forever!! I have used it for gravies, soups, etc...It's tasteless so it's very versatile. The rhubarb/splenda/thicknthin mixture sounds great but I would have to add a large dollop of fresh whipped cream on top to make me happy!!