Greg, nice mod.
Sort of along the lines of what Roger wrote. I have an EE RevX 40 degree quilt in short and slim size. In hindsight, i really should have went with the wide version or at the very least the regular width.
Anyways, i decided to mod my quilt for other reasons besides drafts, but it so happens to work for drafts too if i'm not using the extra Apex material. I sewed a piece of M50 fabric to the top of the quilt, and was surprised to find that it only added very slightly under 2 oz. and that's including 6 smallish sized strips of velcro (which for your purpose, you wouldn't need).
Also what's nice about it, is if i don't need the extra draft protection because it's warm out, or what not, i don't get in the sleeve, but have the M50 material on top, i have extra splash protection when i'm under a tarp. M50 is pretty water resistant for a still somewhat breathable fabric. The water would also have to go through the next layer of 30D DWR nylon. This sort of makes it like a bivy in a way.
(the main reason why i added the M50 fabric, was so i can add or take away a layer of 2.5 oz per sq/yd Apex depending on need)