You might want to contact Ron at MLD - I was looking at having him make me a larger poncho tarp that would be suitable for a hammock. I believe he has one of the best hood closure systems out there.
My basic design was an 11x6 hex hammock tarp with snaps to both fold up the hex ends as well as to snap the sides for when it was in poncho mode. I was also going to have the front of the poncho about a foot shorter than the rear.
I've used a Gatewood Cape and liked what it did to my overall pack weight, but I've since switched to hammocks and wanted to get the same benefits of dual-use. I just haven't pulled the trigger on this yet.