Whatever the design of the joint will be, it is probably being done these very days by skilled craftspeople, separated from our curious eyes by just a single layer of canvas tarp flapping in the breeze at LHR.
Really it would be great if there was somebody who could poke their head under the tarp for a few minutes at LHR, or surreptitiously take a peek at a drawing or two at Boeing's design office.
Given the amazing amount of "inside" information that sometimes pops out on this website, I am really hoping for something on this quite soon.