Freight may be carried on a pax a/c without removing the seats - special holders can be fitted over the seats to accomodate and appropriately secure the cargo. A cabin crew member is also usually carried to confirm that the cabin is secure for take off and landing.
Only company I know of practising this system is bmi on the night mail run LHR-EDI-LHR with the Airbus A321.
Don't know if this is possible on the B777......but hope to find out when tranferring fleet.
