There are some hi-zoot gear makers, expedition outfitter types, who mix films like this on the same surface. Example: A waterproof foot to a sleeping bag, because the foot end is more likely to get wet, not so different than a rubberized (and stronger) bottom for a soft backpack.
So, rather than worry the limited permeability of this, why not a quilt being sewn with two fabrics on the same surface? Leighlo, perhaps under the influence of one of the members of her company's board of directors, <smile> used to make quilts with both synthetic fill and down. (Synth on the sides?) Rab may have made down camping quilts with foot-boxes in less-shifty, less compressible synthetic.
Maybe, in this case, the center of a quilt, above or below the body would be of Thermoflect over down, and the sides would be synthetic fill / Climashield, enclosed in quite-breathable nylon. Water vapor would be kept out of the down, migrating through the synthetic-fill sides, but the whole would not feel clammy.