How to sign hours on Logbook if doing aerial work in right seat on single pilot aeroplane?
I am not sure about EASA regulations but in the US you can log PIC time whenever you are the sole manipulator of the controls and properly rated to fly the aircraft.  In the US there is a distinction between the legal PIC (person legally responsible for the flight) and PIC time which is the person actually flying the airplane.  Since the plane is certified for only single pilot operations you can only log PIC time not SIC time and only when you are the sole manipulator of the controls.  The FAA regulations for logging time can be found in FAR 61.51.