There were a couple 787 tail barrel sections parked outside the office building today. Comparing those to the published pictures of the Ethiopian airplane, notice forward of the aft door, there are a few windows and then a 'missing' window? The 'missing' window is the break where the fuselage pieces are joined. At least externally, the worst of the fire damage appears to be aft of that joint.
Again, not knowing what the internal damage looks like - I'm leaning towards the most cost effective fix being to replace that tail barrel section. Either on site at Heathrow, or if the GSE is too complex, a temp fix to get it back to Everett or Charleston where the tooling is available.
Even if it's a $100+ million price tag, compared to a new 787 it would still likely make economic sense (recall the A380 $150 million/18 month repair bill after the Qantas turbine failure)