Here's something really neat I have just been introduced to...

It's the JakPak. Very similar to the Hilleberg Bivanorak. But, this will unzip around the waist and allow one to wear only the upper half as a rain jacket.

http://jakpak.com/

Anyways, check it out. Looks to be somewhat promising. It looks as though you could put your sleeping bag inside and then slide a non-ridgeline hammock through and use as a waterproof overcover-undercover-hammock sock thingy.

Enjoy!