Maybe the 787-3 wasn't so far away in the first place...
Would a shorter 787 not solve the problem? Perhaps 767-200 length? Then add the clipped wing from the 787-3; with a much reduced range, and hence fuel load, weights would fall dramatically.
Given the carbon structure of the 787, I guess it would be 'relatively' easy to reduce the weight through less tape, so lightening the whole structure.
The downside is the high construction effort...autoclaving the fuselage would be costly. But it might be the right airplane?