I have no idea how Boeing will do the job, I´m quite shure even Boeing doesn´t.
On the ´Dreamliner´-Homepage Boeing states :
" The airplane is super-efficient, using 15 to 20 percent less fuel on a per passenger basis than today's airplanes of this same passenger capacity. This makes the 7E7 as fuel efficient per passenger as today's most fuel-efficient wide body airplanes - the 777 and 747. This is really significant.
The efficiency improvements come from advances in four areas: engines, aerodynamics, materials and systems. The new engines being developed for this airplane offer about a 10 percent improvement in efficiency. Using super computers, we can design the airplane to minimize drag, making it even more smooth and streamlined, which provides another 3 to 4 percent improvement in efficiency. The wing area of the 7E7 is about 10 percent smaller than its much older competitor. The new shape of the wings and raked wingtips is streamlined and strong, yet flexible and light. More durable engines made from advanced and lighter materials provide another 3 to 4 percent improvement in efficiency. And the introduction of more efficient on-board systems provides the final 3 to 4 percent of improvement. It is the combination of these factors that give us our 15 to 20 percent improvement in efficiency. That also means this airplane costs about 10 percent less to operate than today's similarly sized airplanes. "
So Boeing seems to neglect the weight savings of the composite fuselage for their fuel economy improvement calculations. I´m quite shure negotiations with potential operators will quickly turn the fuselage to conventional metal, if it doesn´t offer a remarkable fuel saving.
I might be wrong ...