Sounds like a cool experiment. I dont think the hammock will be in danger because it will be inside the shelter unless you somehow catch the plastic sheet on fire. Fire safety aside I think one of the biggest issues will be keeping the fire going. Big enough blaze to keep it going most of the night will be a fire hazard. Kept small enough to be safe will require regular attention. If you are getting up regularly enough to pee then you should be able to keep the fire going. If you oversleep a bit and the fire dies down to much you will wake up on cold person.