I also prefer a synthetic bag, and I even considered a Pendleton blanket like you're mentioning here (to augment a lighter sleeping bag). The synthetic bag just gave me more bang for my buck, and was much more packable. A wool blanket will be warm down to about 50ºF, and for half the weight, a synthetic like the Mountain Hardwear Ultralamina will keep you warm to 0ºF

I think the only real reason to do the blanket is if you have a leather seat on a bicycle produced before 1980. In that case, roll the blanket and strap it to your Carradice because you're twice the man we'll ever be, cowboy.

Cheers