How about taking a wool blanket and sewing a long zipper on it? Unzip for blanket use, zip for sleeping bag use. I know you can buy these things commercially made out of fleece (from LLBean, and from Coleman, for two examples). I haven't seen any commercially made out of wool, but I'll bet they probably are marketed by somebody, somewhere. Simple enough to make your own however (provided you have a sewing machine and can find a source for nine foot long zippers!)