QFInsider, it is ONLY the "flight deck duty" limit which is affected by where the S/O might happen to find him/herself on the aircraft or indeed anywhere Charlie Q may wish to send him/her.
The duty clock starts running at scheduled or actual (if later than sched) sign on time, end of story. Early Antarctica charters had an early sign on (paid) to allow for the cold weather survival briefing before the flight, if I remember correctly.