Have you thought of perhaps an oilcloth for the outside and the wool for the lining? Would not necessarily be 100% waterproof but I think it would go a long way to being water resistant? Perhaps even add a hood to whatever you come up with for the poncho...for those really heavy downpours?
BTW - capes typically also have slits for the arms (vs a sleeve) as well as a full opening in the front with one or more closures down the front.