I have never tried a baking bag. I know it needs to be a clear bag, and the more vegetation you can cover, the better. I would try it if I were you. Just put the bag over the vegetation and seal the opening around the branch as tightly as possible (I use a rubber band or tape). To retrieve the water you can either penetrate the bag with a small hole, and then cover it with tape, or open the bag and take it down. I have heard of using surgical tubing as a straw as well, like in the sketches of a solar still. Essentially this is a solar still, extracting the moisture from the vegetation.

Does anybody have any actual numbers on how much water a meter of fabric can yield?

I would guess it would depend on many other variables, not just the size of the material.