Well pardon me with another newbie idea. But why not make a water proof sleeve? I imagine my hammock with a bug sleeve, and if it rained I could then just pull the water proof sleeve up and over, instead of a staked out tarp.
I already have an 8 foot long tent made from an 8x10 sil-nylon tarp. Whay can't I put the ridge line through a hole in the back and use it as a rain sleeve?