With regard to fuel temperatures, Flight Planning on the B767 on long sectors (Heathrow-Vancouver/Seattle) often required the cruising height in the latter part of the sector to be reduced to maintain he fuel temperature above the minimum.
The B777 however, on simular sector lengths and OAT's didn't seem to have the same problem. The fuel never seemed to get as cold as it did on the B767.
At least the data will be available on the fuel temperature and one would assume some fuel remained (on board) on which tests can be carried out.