Hammock and a bivvy

I think trying to use a bivy sack in a hammock would be a disaster in a downpour – your hammock would collect the water and you'd be sleeping with your rear end underwater in no time.

The hammock bivy in your link would certainly be a better option, but a hammock hex tarp works really well and provides you a lot of flexibility. You can easily put up just the tarp to enjoy a sheltered lunch. I was on the trail a couple of years ago with two tenters and they kept putting their gear under my tarp in the morning as we packed up in the rain.

Look at ZPacks.com for some light tarp options. Their Asym Diamond hammock tarp weighs only 4.3 ounces (plus guylines and stakes) if you're looking for a lightweight minimalist coverage solution.