Hi, Matt, how are you doing these days?
I've been wondering about this, too. I currently use a Sea-to-Summit Ultra Sil dry bag for my DIY gravity filter. The stiff collar is great for collecting water and the bag can be hung by the closure strap. I haven't had problems with water weeping through. My reservations about using a cuben dry bag center around the possibility of puncture when the bag is hung from a tree branch, especially if it's hanging against stubs of previously broken-off branches. I also await more field reports using a cuben bag!
I'm especially interested because since my dog Hysson contracted a bad case of giardia a couple of months ago (he "exploded" all over the car, necessitating much nose-holding the rest of the way home and several hours of cleanup after we got there), I'm now filtering his drinking water, too. Any weight savings on the cuben would allow me to use a bigger bag!