I offer eVent footboxes, which can be pretty effective, but I wouldn't recommend a Cuben footbox with a breathable body.. The concern is, if you block the footbox moisture inside the quilt, it's just going to migrate to other areas, and get trapped there.
I actually recommended using eVent for the top strip to the customer, but I think he made the right choice with the cuben. Since the cuben is completely waterproof, it's not going to absorb any of that moisture, and since it doesn't have any protruding fibers, I think most of the moisture is going to bounce right off of it. It likely will form some condensate on the outside, but at least it will not soak the insulation, and can be wiped right off. Since the moisture vector is from outside the quilt, it shouldn't migrate inside.