I'll second the bivy concept. But in reality, why do you want to waterproof it? If you use a propper shelter you should not need a waterproof bag. If you use a mini tarp then maybe the foot box, but even then, making your mini tarp larger would weigh less than the W/B fabric foot bivy. A little mist is not going to hurt your bag unless its realy cold. And if its realy cold, a better shelter is needed.
I'm not up to date on what materials are used, but they are not normal gortex, they breath better and somehow are just as waterproof (?).
I had superhuman powers, but my therapist took them away.