I don't think you quite understand the role of a Cruise relief pilot.
Im pretty sure they are paid less as thats the whole point of having a non-type rated crew member, so they can increase the profit margins.
They will undoubtedly only occupy the FO seat for a few hours during the cruise. Not exactly flying the aircraft is it!?
They will have to keep their licences and ratings valid, not sure whether its at their own cost or offered by the company?
Nick