I regularly use a tarp that is shorter than my hammock without problem. That's because I bring the foot end of the tarp down toward the hammock and center the tarp carefully, because the hammock in use is only 5/6 its length, and because there are DWR fabric hoods over the hammock ends. Some side-blow water falling on the ends does not get channeled into the hammock.
Methinks more folks going for very long hammocks should think of lightweight DWR fabric hoods, maybe as part of DIY bug nets and at least for the foot ends if they think such hoods too enclosing for the head end. A hood sheltering two feet of hammock may require no more than 1/3 yd sq of fabric, if you are calculating weight.