Google up the Moonbow Gearskin. You're almost there.

I think the guy in the hammock would stay dry. If it got more windy he'd have to pitch the tarp closer to the hammock and make it more A frame rather than leave it open that way. Easier to do with a cat cut tarp with four tie outs than a diamond tarp, if it's real windy.
"In the beginner's mind there are many possibilities. In the expert's mind there are few." Shunryu Suzuki