A 777 has a lot of space up top that isn’t being used for anything beyond crew breaks; overhead is also rarely a problem on long long hauls (the planes are bigger, have plenty of overhead space per seating, and people don’t carry as much on when they have to go through immigration and customs anyways). I don’t think space is really as much as a problem as weight, which shouldn’t be an issue here.
I don't think weight will be a major problem here, given the stated focus on it.
I couldn't find weight specs for units of Zodiac Z535s or Recaro PL3530 seats, which are two of the major competitors in the premium economy market to compare with the "under 50kg per unit" claim on the Zephyrs. If you have the numbers, I'd be interested
