It has always been my understanding (haven't looked it up in the EASA rules yet) that duty and rest limitations apply as soon as a pilot receives "renumeration". No matter if he flies commercially, private or in the training sector.
Can anyone confirm this? As far as I know, part NCC just have to have/implement a fatigue management plan.
Id really like to know what other operators are doing with respect to this. We have flown private with the owner of the aircraft whilst otherwise over duty hours if it were CAT OPS with a release certificate signed by all crew and the DFO or fleet manager.