The first time I rode in a jumpseat for a takeoff was in a Lufthansa A300 from the (then) new Munich airport to LHR.
With only a dozen pax and not a lot of fuel we pegged the VSI when ATC asked us for "Absolute maximum best rate of climb now, please".
I've found the best way to get into the flightdeck is to ask the senior cabin crew member to pass a very brief, polite letter to the flightdeck with a business card attached.