When an air traffic comntroller is on my flight, and had either made himself known (or the ATC unit has done so, for him), they are invited to the FD, assigned the number one observers seat (if they so desire) and are allowed to remain....for as long as they like, takeoff and landing included.
And yes, I have full and complete authority to do so at my airline, as I am the Commander....and what I say, goes.
And no, we don't have some government official telling us what we can (or cannot) do...in fact, if we do have said government official on board, they are offered the same privileges as might be extended to ATC folks....including a seat in the business class section, if we have one available.
This is called....common courtesy.... something I otherwise find lacking in many airline operations, today.
Yes, we are old, and yes we fly an old (but superb) widebody airplane, however we have not forgotten how to treat folks with the respect they deserve.