"I thought seriously about the cape .... .... But I'm also considering a custom 6 x 9 Cuben poncho"
I've lusted for a Zpacks hammock tarp @ 6.5oz, but I need a $250 windfall to go with it :)
I've thought about a poncho with "wings" that fold under, giving nearly twice the width. The back could have some extra length too.
I have the XL cape which is 137" x 120", giving pretty good coverage @ 12oz. It would be great knocked off in Cuben. I picked up a Backpacker model Hennessy that makes a light combo with the cape. It's a bottom entry model, which is useable, but I don't care for it much--- I'm waiting until Winter to have 2QZQ add a zipper.
I use an undercover and the foam pad from the Hennessy SuperShelter along with a space blanket for the UQ. Your differential cut/layered IQ is a good idea. I have one undercover that is also a poncho, which works well.
I don't think I would fit well in your rig, but it *is* light.