Polypro pellets are washable-those are the kind used in stuffed animals. If you could rig some kind of pockets for the pellets, you could take them out (whatever they are) and wash the 'skin' of the blanket, if that makes sense. Maybe you could use something like pantyhose/tights to make tubes of pellets. In case of an accident, they could be removed and hand washed, without putting them in a washing machine. (I always have visions of pellets everywhere in a washing machine.)
My husband has horrible, chronic insomnia. In desperation, I bought him a weighted blanket-a vintage twin-size wool crazy quilt, batted with raw cotton. It weighs like a dead horse, and works like a charm, but isn't washable.